There are many reasons for slowness in the Library module, but having 75K images should not be one of them.
I can think of a few possible reasons (maybe there are others)
- Lightroom is generating previews
- Lightroom is doing facial recognition
- Lightroom is uploading photo to Lightroom mobile
- Your catalog needs to be optimized
So I generate previews when I add images to the catalog; I don't do facial recognition or upload to LR Mobile. I have optimized the catalog repeatedly. Like I mentioned above, I have even rebuilt the catalog from scratch. It is the same issue on different computers. There is something about the catalog that is slowing searching down.
You don't do facial recognition or upload to LR Mobile, but you need to check that you haven't turned these on accidentally.
When you are not doing anything in LR, not even the searches, is Lightroom using a lot of the CPU or Memory?
What is your operating system (exact version please)?