The latest TF2 update seems to cause a problem with custom HUDs in MvM. Medics experience severe lag when hitting bots with the projectile shield. I'm able to reproduce this using IdHUD 3.5, but I suspect other/all HUDs are affected since no one commented about also using IdHUD when I mentioned it. [edit: ... to other players who had the same issue ]
I found a comment on this thread from 10 months ago, just prior to the 3.0.7 release http://107.191.99.83/16751/improved-default-hud/?page=5#127 reporting an identical issue. Same bug back again?