I'm sure this problem you describe is down to the glacial pace Thunderbird now does anything (it is sometimes taking 20 seconds or more to open an email tab now for me), but just to be sure, in Settings->General, under Reading & Display, have you got...
I have 2 gmail accounts. One works ok, but my primary one is incredibly slow. It is sometimes taking upwards of 20 seconds to open an email. As far as I can tell all server settings are exactly the same. The slow account has a much much larger nu...