Preview App became too slow

You are aware that the system caches are intended to improve system and app performance?

Do you have any add-on cleaner apps, add-on anti-malware, add-on anti-virus apps, add-on VPN clients, add-on network firewall apps?

If so, remove all of those per the vendors’ instructions, reboot, and try again.

Some of these apps can and have caused system corruptions, as well.

At best, rebooting might clear this corruption.

At worst, some of those temporary files were needed by whatever services and apps were in use.

I’d suggest practicing on a scratch Mac first, before testing on your production Mac.

And I wouldn’t, and don’t, suggest wholesale deleting temporary storage.

macOS generally cleans up itself, and when it doesn’t, the re-install while recovering your files from backups works quite well.

You’ll want to have a Time Machine backup or two running here, too.

This on the off chance your testing irreparably damages your login, or maybe (less likely) your install.

Or should you accidentally delete a file you need, or should some app corrupt one of your files.

ps: if you’re just messing about within your own login context and not within a system-wide context, and have the urge to experiment, creating a second login and logging into that and using that for your testing can work as a testing location. So long as you stay within your login path and avoid invoking root privilege, any potential damage will likely be isolated. But have backups, regardless.