I can't speak for mrweasel, but in my experience, TortoiseHg struggles with performance on large repos, with basic operations (like refreshing the list of modified files in the Commit window) taking many seconds to complete. It consumes a lot of memory while idle (around 500MB on my machine), often becomes unresponsive or crashes, progress bars only represent the lowest level operations, not overall progress (eg: checking out a repo containing subrepos results in the bar going from start to finish several times), etc, etc.

It's not terrible by any means, but I'm interested to see what other clients can bring to the party.