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.
I spent months trying everything I could think of: downgrading to Catalina, turning off transparency/shadows, running as few background services as possible, and not using scaling at all (which was the most effective solution). And this was only with 2x 4K monitors; I added a 3rd more recently.
Nothing worked. Thermal throttling and insufficient sustained power were two problems I was able to identify (the 96W adapter is not sufficient for the system's peak power load, so it uses the battery to get over 96W of draw).
Eventually, I broke down and bought an eGPU (Blackmagic eGPU) which solved the problem. For about ~$700, I'm now able to use my machine without a hiccup. Not a great or affordable solution, but it has made my $3,100 machine usable again.
I’m pretty sure my i9 model has the lower end 5300M.
Another question: what are you using to connect the monitor to your laptop? USB-C to DisplayPort, here. I formerly used HDMI off of a USB-C hub but it was a bummer.
It’s pretty bad the Apple still denies any issue, not being able to use an external monitor at all through covid suuuuucked
