My account experiences the slowness the OP describes:
- Seconds to open a conversation, often punctuated with messages indicating that the operation failed and must be retried.
- Sending mails fails, and must be retried multiple times.
- Delays correlate loosely with US workdays (I'm in the US). Performance is consistently better at 1AM EST than at 2PM EST.
- 0 connected apps.
I haven't done extensive comparison to other accounts, but I believe that the number of messages is a cause of sluggishness. My mailboxes are > 120k messages. This causes problems with many mail clients; for instance, Sparrow (iPhone or OS X) is useless.
I'm surprised that the GMail team hasn't figured out how to handle busy mail users, given the propaganda that all Google users use GMail. I'd love to have the option to pay for Day 1 performance in my GMail boxes.