Do you see the same thing if you restart your computer in safe mode with networking? I would hazard a guess that the issue is not Thunderbird but other third party software the pokes it's head up when Thunderbird is running. (Think antivirus because it functions slowly or VPN that may restrict bandwidth on email connections because the operator sees folk routinely download gigbytes of data from mail accounts, change a setting and repeat, even google will restrict your bandwidth if you use to much in a day.)