That's just Evolution. It's a big, complicated turd that uses a thread pool with no priorities. So you can sit there waiting to read a message while it is blocked checking your other folders. Also it's keyboard shortcuts are stupid.
Unfortunately, Thunderbird does not like to talk to Microsoft Exchange Servers in their native tongue. If it weren't for that, or if my employer did not run on Exchange, I would be using Thunderbird already.
Thunderbird is a lot better.