Connecting an external monitor to a laptop should increase gaming performance in most instances, let’s discuss why this is the case, and find out just how big the difference is in a number of different games.
Most laptops these days have Optimus. This basically means that the lower powered integrated graphics within the processor are used to send display signal to the screen. This has the advantage of improving battery life, as they’re more efficient and require less power to operate, however the downside is gaming performance kind of sucks when compared to what AMD and Nvidia have on offer. These higher powered discrete graphics options basically sit idle when there are no GPU intensive tasks running on the system. If you start up a game though, the more powerful discrete graphics will be used to render the frames, however in an Optimus system the discrete GPU sends this output via the lower powered integrated graphics.
Depending on the game and settings, this can result in a bottleneck situation where the integrated graphics reduces gaming performance.
If your laptop only uses the discrete graphics and doesn’t have Optimus, like the Aorus 15, or allows you the option of disabling Optimus after a reboot, like the Lenovo Y540, then you’re probably not going to see a performance difference with an external screen as you’re already bypassing Optimus. This also applies for current laptops that have G-Sync, as G-Sync requires a direct connection between the screen and Nvidia GPU to work.
Many laptops that have display output ports such as HDMI or DisplayPort have them wired directly to the discrete graphics, meaning that by running an external monitor connected to one of these we’ll be bypassing the Intel integrated graphics and Optimus as a result.
This same idea should also apply to AMD Vega integrated graphics. There will be no difference if the port is connected to the integrated graphics though. To demonstrate this, I’ve tested the Eluktronics MAX-17 gaming laptop with and without an external monitor connected. All testing was done with the same Windows updates, Nvidia drivers, and with a 1080p resolution. Literally the only difference is that I’ve tested either with the laptops screen, so Nvidia RTX 2070 pushing frames via the Intel GPU to the laptop’s display, or with the external screen connected to the laptops HDMI port which skips the Intel GPU and connects straight to the Nvidia RTX 2070.
It’s important to note that when using an external screen, you’ll still get low FPS if the laptop's screen is in use because the Intel GPU will still be active. To avoid this, either close the lid of the laptop, or most laptops have a shortcut key you can press to select which display to use, so use that and just pick second screen only, you can also just do this through Windows too. This disables the laptop screen without the need to close the lid, which is useful as some laptops require the lid to be open to cool properly.
If you’re not sure which GPU is in use, you can simply check the task manager performance tab to see which are active with a graphical load running. 
Anyway with all that out of the way, let’s get into the gaming benchmarks. These are the results from Red Dead Redemption 2 which was tested with the games benchmark tool. 
I’ve got the laptop screen test shown by the purple bars, the external monitor connected over HDMI in the red bars, and I’ve tested all setting levels. In this particular title there wasn’t too much of a difference, but the external screen was performing better. 
Battlefield V was tested in campaign mode, and this time there was a bit more of a difference between the two, namely at low settings where the external screen was 13% faster when compared to just using the laptop screen, and this will be a trend we’ll see throughout most games tested where lower settings see a bigger difference. 
Shadow of the Tomb Raider was tested with the games built in benchmark, and there was a fair 7% increase to FPS with the highest setting preset, however lowest settings saw the biggest difference, with the external screen now almost 21% ahead of the laptop screen. 
Control was tested walking through the same area of the game in both instances. There’s a smaller 4% higher average FPS at high settings with the external screen, and then this is a little closer to a 5% boost at low. 
Call of Duty Modern Warfare was tested in campaign mode with either maximum or minimum settings, as it doesn’t have built in setting presets. This time there was a larger 11% performance improvement to average FPS at max settings, a nice difference for such a simple change. 
Apex legends saw basically no difference at minimum settings, as the 144 FPS frame cap was being hit consistently with this hardware in either setup, however there was a small 4% improvement to average FPS at max settings. 
Borderlands 3 was tested with the games benchmark tool, as there’s no frame cap in this one we’re back to seeing larger differences at lower settings. At very low settings, the external screen was 25% faster compared to using the laptop screen, however maxed out there was less than a 3% boost.
Fortnite was tested using the replay feature, with the exact same replay file used for all testing. High and epic settings saw small differences, but there were much larger changes at lower settings, low settings for instance was 38% faster in average FPS with the external screen. 
Dota 2 was tested playing in the middle lane, and there was a fairly large 20% higher average FPS with the ultra setting preset on the external screen. The external screen was ahead at all setting levels, but in this test it seems to do better at the higher settings. The difference between 1% low in this game was much more minimal in comparison. 
Assassin’s Creed Odyssey was tested using the built-in benchmark, and this test saw an 8% higher average FPS with the highest setting preset, with a much larger 33% improvement at low settings, where even the 1% low with the external screen was close to the average from the laptops screen. 
The Witcher 3 saw pretty minimal differences between the two, but this wasn’t surprising as I’ve tested this one in the past, though there was more of a difference at the lower setting levels where frame rates are higher and presumably where we’re more CPU bound. 
Ghost Recon Breakpoint was tested using the built-in benchmark, and the results were similar at the higher setting levels where there was minimal change, however there was a much bigger differences seen at the lower levels. 
The Division 2 was also tested using the games benchmark tool, and once more there was a much smaller performance difference with the higher setting presets, while the external screen setup was almost 19% faster at low settings. I’ve tested CS:GO with the ulletical FPS benchmark. 
This game saw larger differences compared to the other titles tested. It seems like the higher your frame rate, the more the Intel integrated graphics is bottlenecked, so for games like this there are huge gains to be had with an external monitor. 
Overwatch was tested in the practice range, so while not quite the same as actual gameplay, it allows me to perform the same test pass more accurately which is ideal for comparison purposes. As a game that hits very high frame rates, larger differences were seen between the two tests between low and ultra settings, especially in terms of 1% low performance. 
Far Cry New Dawn was tested with the games benchmark tool. There was a fair improvement to both average FPS and 1% low performance with the external screen, with almost a 10% boost to average frame rate at ultra and slightly smaller 9% improvement to 1% low.
On average over these 16 games with the highest setting preset available we can see that using an external screen was around 6.6% faster than the laptop screen. 
The results of course vary wildly based on the game, and even the specific settings, so let’s also check out the differences if we look at minimum settings.
With minimum settings the external screen is now averaging a 16.9% higher average frame rate, so it’s doing much better compared to max settings.
As discussed along the way, this is because the processors integrated graphics seems to become a larger bottleneck as we hit higher frame rates, which is what we typically see at lower settings that are more CPU bound. A part of it could also be that the iGPU now also doesn’t need to waste system resources.
It’s possible that for the same reason we could see less of a difference between laptop and external screen with lower specced machines. Lower specs would give us lower frame rates, so the bottleneck may be a little less pronounced with lower tier hardware.
I had a quick look at temperatures, and didn’t see a noteworthy difference in either configuration. 
So to conclude, if your laptop has Optimus and you’re not able to disable it, which is the case for most gaming laptops I’ve tested, you can get a decent performance increase in most games simply by connecting an external monitor. This is especially true the lower settings you use, but even higher settings did see a benefit, it just wasn’t quite as big. It will of course depend on the laptop though, if the external display outputs are wired to the Intel graphics then you’re out of luck.
Of course, I’m not suggesting you take an external screen with you to get this performance boost, but if you also use your laptop at home you could easily dock it to an external screen and get a better gaming experience, not only in terms of a larger better looking screen, but also higher frame rates as we’ve seen throughout this testing. So now that you’re aware of this, is it going to change how you game on your laptop? Let me know if you’re going to get or are already using an external monitor down in the comments.
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