Hi Nicolas,
I apologize about that, I was focusing too much on the email part and not the overall post.
That is definitely interesting, we can eliminate that as the cause. I have just tried Espionage with Postbox Express, no issues there.
If you still have Postbox with Espionage; please run a diagnostics report for us after running PostBox for a few minutes, so that we can see what’s going on in the background. You can download our diagnostics tool from here:
http://www.taoeffect.com/other/TEDiagnosticTool.app.zipIn the Diagnostic tool, when it ask for the description of the problem, add “Forum: Thunderbird slows under Espionage”, so that we can connect the dots.