Will an external 4K monitor cause performance issues on my MacBook Pro?

I am using a Macbook pro 13 2015 with a 4K 27 inch display, and definitely having performance issues when I use it in one of the scaled resolutions (e.g., 2560x1440, 2304x1296, 3008x1692..). When used in "Default for display" mode, which is an equivalent of 1920x1080 UI resolution, performance is great.

According to specs of 2015 Macbook pro 13, the limit is to drive a 4K display at 3840x2160 resolution. When Macbook pro is used in any of the scaled resolutions, internally, GPU renders a canvas which is much bigger than the actual 4K resolution, and then downscales it to 4K. This puts real burden on performance.

The performance hit shows itself as stuttering animations on mission control window animations, with addition of fans spinning on higher RPMs.

However, when I look at specs of 2017 Macbook Pro 15.4 which is listed here, I can see that it can properly drive two 5K resolutions simultaneously. Therefore, if you use 2560x1440 scaling on 27 inch 4K monitor, we could expect that its GPU would easily render the 5K canvas, then downscale it to 4K native resolution without any problems.

This was the elaboration about performance. From sharpness point of view, 2560x1440 scaled resolution would be just a bit softer than the exact pixel doubled 1920x1080 mode, however, if you look at a proper distance (greater than 53 cm according to this site), you won't notice too much difference. 2304x1296 scaled resolution works even greater, because text is a bit bigger.

If you accept to spend more money and if you can find one, the LG Ultrafine 4K or 5K displays from Apple would be the best, because you would be able to use them in exact pixel doubled mode, which gives sharpest image and best performance. However, with much less price, and some acceptable performance penalty, you could as well live with a 27 inch 4K display from other manufacturers. Just be sure to get a model which has less compatibility problems with Mac.

You can also have a look at my answer to a similar question here.