One should use Firefox browser 64bit for 64 bit OS. But you can install 32bit Firefox on 64bit OS but not in reverse. As to load times there are many factors from RAM to OS to Addon installed on the browser or AV checks causing loading issues so there no one reason why. This will be a computer shop to find the load issues.

My PC is 64 bit. Should Firefox be 64 Bit, too?

How do you figure your Firefox install is 32-bit and not 64-bit? https://www.mozilla.org/firefox/all/desktop-release/win64/

You can run the 32-bit Firefox on 64-bit Windows though for macOS there is no 32-bit anymore and for Linux it may be a hassle to get the 32-bit Firefox running on 64-bit Linux and no benefit to do so.

Mozilla did not have 64-bit (Wind4) Firefox for 64-bit Windows officially for Release until Firefox 43.0 released December 15, 2015