will installing Big Sur over Mohave slow my Mac

I suggest you try the following and find out yourself:

1. Make a bootable clone copy of your current macOS Mojave system drive onto an external USB drive. I suggest using Carbon Copy Cloner for this purpose.
2. Verify that you can boot up from this drive.
3. Once verified, go through the installation process to install macOS Big Sur onto the internal drive.
4. Try it out to see if you notice any performance issues. If not, you're done.
5. If you are not satisfied, restore the clone from the external drive back to the internal one.

I can't notice any speed diff between the two on my 2019 iMac.