Have you enabled an appropriate level of swap for your system usage?
I almost always tweak the default swap install, especially on older/resource-limited boxes; or when I forget the moment I notice performance is less than I expect. The default install provided by calamares is too small in my opinion, but we cannot change that (yet).
I’d also check the health of your drive, a new installation will likely have been placed on a different part of the disk you were using before, so exploring system logs look for clues can be useful, plus the usual SMART checks from live media I perform on any unexpected behavior with a box.
You didn’t mention what apps, which does matter. If it’s a snap for example, the initial run has a lot of setup/unsquashing to perform before execution can start, if the app is a GTK app those libraries/toolkits will need to be loaded into RAM as your desktop uses Qt etc…
From what you provided - I’d start with system logs, then the hardware checks (but that’s just as its something I do on any unexpected issue…).