Based on a personal experience, it seems to be some sort of data fragmentation problem. I had similar problems with Gmail a few weeks ago - sending a message w

Based on a personal experience, it seems to be some sort of data fragmentation problem. I had similar problems with Gmail a few weeks ago - sending a message would take ~30 seconds, accessing labels took up to a minute, messages were appearing in the inbox 30 minutes after they arrived, etc. The slowness persisted for about a week. 2 weeks ago my account went down for about 10 minutes. Accessing mail.google.com while logged into my Gmail account would return a 503 error; my backup Gmail account, which had now slowness, was accessible. 10 minutes later, I was able to access my account and the slowness was gone. My guess is that Gmail took my account offline while my messages were re-indexed.