When scanning headers and/or reading messages there are long delays with OS pop up saying Thunderbird is unresponsive. It typically happens when downloads and copying are occurring in the background. TB seems to be giving too much priority to the background making everything in the foreground slow and frequently stop completely. When composing a message a typed line does not appear for 20 seconds or more making it difficult to prevent typing errors. I have not had this problem with older versions of TB. My local message store is well over 20 GB. I expect some operations to take awhile but none should slow or stop response to immediate entry. When TB logs into servers and there are network problems input at the keyboard is often ignored completely. To regain control I have to call the task manager and do end process and restart TB. At first I thought this was a problem with one computer. I have several with a mix of several versions of Windows (7,8,10 and 11) with several email addresses and have the same problem in varying degrees with all of them. If this is a setting problem I have no clue how to fix. I have been using TB more than a decade and feel confident about how to do most operations. I'm using version 140.3.0esr (64bit).