MacOS in particular does an excellent job of rendering 2560x1440 to a 4K screen, and the increased DPI over a regular 1440p 27" screen is very noticeable.
Another option if you're not a fan of 27" displays is a pair of 24/25" 4k screens which can be run at a scaled 2304x1296 resolution. This still provides a decent amount of space without text being too tiny. Alas, 4k monitors <27" are increasingly rare these days.
MacOS in particular gets amazingly slow when you don't use a 1:1 or 2:1 scaling. I too have have two 27" 4k screens and they made the machine unbearably slow. It got so bad that I now treat them as 1440p screens and let the screens do the scaling. It's not pretty and slightly hazy, but at least the machine is usable.
Also sometimes the virtual memory subsystem seems to get confused or overcommitted (e.g. after running a bunch of large VMs in VMware) and from that point on everything is just slow, even if you shut down every VM to relieve the memory pressure. It may be related to macOS's use of memory compression.
Then there are various background daemons (mdworker, syspolicyd, photoanalysisd, etc.) that occasionally wake up and decide to eat all of your CPU while simultaneously hammering your file system. The only effective response, short of disabling the offending service (which is much harder than it is on Windows or Linux, due to SIP) seems to be to let them run their course as they decide to scan every file they can find for the 100th time.
And when your laptop heats up, then macOS starts throttling the system via kernel_task processes that appear to be using all of your CPU.
My work laptop is a 2019 i7 MBP, it struggles with my 4k monitor regardless of scaling. I bought the cheapest mac mini last year to see what the M1 fuss was about, and it has no problem with the 4k screen, even with scaling.
Other OS's? Windows is passable until you start transitioning in and out of full screen. Linux...
If there is a speed decrease, I can’t notice it on an M1.
I’ve seen a lot of people parrot this claim or claim it renders awfully but have yet to experience any evidence. On the contrary, it’s been glorious.
Edit: if you do 1440p scaling on a 4K on macOS make damned sure you select “1440p (Hi-DPI)” other you get a pixelated mess.
I don't "parrot" the claim. I've experienced the problem. It's day and night. After installing Monterey I couldn't run MS Teams on the external monitors anymore. It more or less locked up and I couldn't move the window back to the laptop screen. This was repeatable.
The whole problem went away when I selected 1440p (the "low resolution" one). It's fugly, but at least I can actually use my other monitors.
32” at 4k (native res, 1:1)
30” 2560-by-1600 (native res, 1:1) ( few monitors support this physically, but two are my goto: old 30” apple studio displays , and a 30” old dell monitor. Both can be found on eBay at very low cost , but do use 2 to 4x the power draw as modern monitors)
I always try to get the company to pay for me of course, but I have no patience for suboptimal equipment any more, so I’ll buy it myself if I have to.
I may try pushing for a monitor refresh again once we go back to the office.
An option that I think might be interesting is 3 displays side by side, but with the center display in portrait mode rather than landscape mode giving overall an inverted T shape to your combined display space.
I find this works well for the "adjustable height desk" systems one puts on top of a regular desk. They usually aren't wide enough to have two monitors with one directly in front of the user. The portrait monitor, if the cables are long enough, stays on the fixed-height desk.
I'm unable to use two monitors side-by-side anymore. Working for hours with my head always turned to one side gives me headaches.
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Edited to add note on desk-placed "adjustable height" systems.
How does this work? Will it just upscale or is e.g. text still rendered at 4K? Rendering not at native resolution results always in blurred edges in my experience.
Here's an album with a pair of screenshots from my own 4k display: https://imgur.com/a/7AHZZZv -- the scaled one is how I normally use it.
The Windows UI scaling slider behaves in exactly the same way, though fewer apps include 2x or 3x bitmap resources.
This makes it sound as if macOS upscales a 4K render when displaying to (for example) 5K monitors, but on a 5K monitor everything is ultimately rendered at a full physical resolution of 5120x2880. But in the Displays Preference Pane, the logical resolution is set by default to 2560x1440 (2:1). One can choose a logical resolution of 5120x2880 (1:1), but I can't imagine anyone working like that.
