I did a LOT of careful QA times testing on 3,0.0 alpha and RC builds before release on my W10 PC and my Bug Sur Mac - on both platforms most things (like saving and opening exporting and effects all run the same as 2.4.2 or a little nit faster.).
The one thing we know that takes longer is Exiting Audacity as the database has to be tidied up.
I did a lot of pre-release testing on this and I am confident that it does work (backed up by a couple of 3.0.0 user reports).
I just timed how long it took Audacity 3.0 to import two stereo MP3 files, a total of 261k, from a digital recorder connected via USB 2. Total time was 2:34:91. Saving the imported files as mono tracks (I split stereo to mono and then deleted the 2 extra tracks) to an 8T Seagate USB 3 hard drive connected to a powered USB 3 Anker hub took 00:41:46. Compacting took 00:11:00. Loading (re-opening) the project was instantaneous. Too fast for me to time.
I’ll test the same files on another Win10 PC that’s still using v.2.x and see if the times are equivalent.