There's something else that hasn't been mentioned yet: Video games in particular have been so detailed since the eight generation (XB1/PS4) that 1080p with its significant compression artifacts on YouTube swallows too many of those fine moving details, like foliage, sharp textures, lots of moving elements (like particles) and full-screen effects that modify nearly every pixel of every frame.

And no, you will not get a less sharp image by downsampling 1440p or even 4K to 1080p, on the contrary. I would recommend you take a few comparison screenshots and see for yourself. I have a 1440p monitor and prefer 4K content - it definitely looks sharper, even down to fine-grain detail and I did the same when I had a 1200p screen, preferring 1440p content then (at least as soon as it was available - the early years were rough).

If you are noticing high CPU usage at higher video resolutions, it's possible that your GPU is outdated and can't handle the latest codecs anymore - or that your operating system (since you're on Linux based on your comment history) doesn't have the right drivers to take advantage of the GPU's decoding ability and/or is struggling with certain codecs. Under normal circumstances, there should be absolutely no increased CPU usage at higher video resolutions.