What difference does HiDPI make to the display VS not using it at the same resolution?

HiDPI leaves the display at is highest native resolution. Pictures and backgrounds are beautiful as can be.

Some Users find that TEXT at that resolution is too small to be viewed easily. so the Mac offers the ability to use SCALED resolutions, which apply ONLY to text. (Apple does not thump you over the head with that subtlety.)

Then TEXT will be drawn larger, then mathematically scaled back to a resolution that is more readable, while maintaining its crisp edges and smooth diagonals. If your scaling is not an integer number, there is a little graphics computation involved, but your Apple-Silicon Mac is a computational and graphics Beast, and should NOT be struggling to accomplish this "in the blink of an Eye".

first things first:

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners. or a VPN that you installed yourself. The main reason is that they are relentless in scanning your files, non-stop, looking for things virus-like patterns in Everything. When completed, they do it all again.

Your Mac should be having NO Issues whatsoever formatting for HiDPI display. That fact that it is visibly struggling suggests you have resource hogs (or worse) installed.

In NOT HiDPI, the whole screen is set to a lower resolution, so text is perfectly visible as rendered (no scaling required).

In HiDPI, scaling would be required for rendering scaled "large-print" text. if you have your computer clogged up doing anti-virus scans, that cuts into its ability to do "real" work.

the most often recommended anti-virus software here are ClamXAV and MalwareBytes, because each can be set to scan only on demand. This stops roobbing your Mac of is top performance, while reducing its vulnerability not at all.

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MacOS shares a lot of the lock-down mechanisms developed for the iPhone. Applications are all sand-boxed with a list of the resources they require, and they cannot ask for anything outside their sandbox without crashing. Signed Applications are checked that they are from legitimate Developers, and Notarized Applications are delivered with the assurance that they have NOT been modified since their release by the Developer.

Recent versions of MacOS completely changed how you should think about malware.

From MacOS 11 Catalina onward, the system is on a Separate, crypto-locked System Volume, which is not writeable using ordinary means. Any unauthorized differences that appear to the crypto-locked volume are quickly detected and you are alerted.

So you could store just about every malware known to mankind on your Mac, and your Mac would not get infected spontaneously. Scanning for virus-like patterns might make you feel a little better now, but it is outdated nonsense.

Nothing can become Executable Unless/Until you supply your Admin password to "make it so".