Including the last one where Paul corrected the comment about Gmail being slow.

Good point. It does seem that Paul B. could be the friend. In addition, PG mentions Paul B. in several places, just a few including:

http://www.paulgraham.com/13sentences.html

http://www.paulgraham.com/5founders.html

If true, it's especially interesting that Paul B. was the creator and lead developer of Gmail and that he corrected PG by stating that "GMail has become painfully slow" instead of "GMail is painfully slow" thereby humorously dissociating himself from the act of having developed something slow.