I don't why kdenlive is getting slower after following the updates.
I have quite fast laptop but still slow, even i only edit 1080p video and used proxy too.
My spec: 64 ram, nvidia geforce 3060, 1 tb storage.
I tried to do the same edit on premiere, surprisingly it's much faster (no delay) and using full resolution preview.
If it's keep on like this, it seems that i have to ditch kdenlive as my favorite editing software.

Which OS?

For linux i can advice to delete the caches once and a while as they float full with unused old snapshots and such. Linux is very good, almost perfect, but also can have some minor lacks like this.

And for windows it also always was usefull to clean up old left sh*t once and a while (and not only for kdenlive).
What i sometimes did on windows in the early days when things got slow...
Delete the software (kdenlive in this case).
Clean all left files (dll's and such).
Re-start the computer.
And then re-install again.
This usually cleaned up a lot of left over sh*t which made things slow.