Comment by Darkskiez

That system was what fed into the one I touched, which was an offline batch pipeline, so I wasn't tooinvolved with the Gmail internals that you're talking about. I know about 10 years ago many Googlers had the same sort of problems as you describe, I assume it's a tiny bit better these days, most of the backend has been replaced over that time. Expensive operations are often slow to ensure there is reasonable isolation between users, so you don't hurt someone elses experience when you do these things. They aren't frequent operations/CUJs so they aren't optimized either.