Well obviously Thunderbird settings have something to do with the problem. For example when I disabled the setting "Enable global search and indexer". I also disabled "Allow Windows Search to Search Messages".It's now a lot better. But it DID lag mas...
Thunderbird 115.6.1 is unusable. E.g. when trying to reply to a message, my laptop with 32 GB of memory and Ryzen 7840HS CPU lags so much that you have to wait like 2 seconds after each key press. It's impossible to write any emails with this app.