I used Mailspring with about 7 accounts (fastmail, outlook, gmail and yahoo) and it routinely fails to sync and I have to reconnect them. There is also this bug I've had for well over a year on windows where if you close the window it cannot catch focus and be maximized again, found a weird trick on the forums where you double click the taskbar icon to get it back. The latter was fine but getting kicked out and missing emails made me find an alt and so far so good with Thunderbird.

Is it possible that the perf issues were related to the number of account/mail?

Unlikely, I don't think I have 10k emails between all 7 and on average I get maybe a dozen or so a day. Even when I had like 3 accounts they would randomly just disconnect and need reauth.