It's the sheer number of simultaneous but different conversations that makes it especially brutalizing.

This seems plausible. One thing I sometimes do (I'm doing it now) is to lock down my switching. I worked all morning on work stuff, now I'm switching over here for a bit and talking to you guys, and I will then switch away for the rest of the night. It's fragmentation, but not terrible.

Hard to be disciplined about it, though. Easy for me to go off the rails.