Running macOS Monterey I've been using TB 128.5.2esr and 128.6.0esr for a while without any slowness or freezing. I just updates to v128.6.1 and for the moment don't notice any slowing down. Did you already test Thunderbird in "Troubleshoot Mode" ? https://mzl.la/3AbU5jz

I tried Troubleshoot Mode by holding down Option while opening Thunderbird and the problem was still happening even within Troubleshoot Mode. Interestingly, it stopped briefly when I opened up Thunderbird in regular mode again, but started back again shortly after and ever since then going back into Troubleshoot Mode and back into normal mode has not helped. I have no extensions or anything special running in the background either and my system is a Mac Studio M1 Ultra with 128gb ram, so its baffling to me why this is happening unless it is an actual bug that got introduced starting with the release right after 128.5.2esr.

My MacBook Pro is still running 128.5.2esr and is working perfectly. I don't dare upgrade it.

Do you use any antivirus software on your Mac Studio? Do you have very big inbox folders in your accounts?

I propose to run Thunderbird in a new test profile with just one account. In TB go to Help > Troubleshooting information > Profiles > About Profiles. The latter will open a new tab "About Profiles" which is some sort of a profile manager. In the upper left corner click on "Create a new profile" and follow instructions in the "Create Profile Wizard". At the end click on "Launch profile in new browser" which will open your test profile in a new Thunderbird instance and prompt to create an account. Now quit Thunderbird in your old profile to avoid interferences with its "beachballing". Back in the new profile create an account and test downloading, opening, deleting, ...etc messages to verify if the same problem as in your first profile happens