UGH! VERY VERY BAD!
WHY are you using nick to join the two tables????
For that matter, WHY are you using nick in the COMMENTS table *AT ALL*!!!
You should REPLACE the nick field in COMMENTS with user_id and make it a FOREIGN KEY to the id field in USERS table.
But anyway, the reason it is so slow is that nick is NOT AN INDEX in your COMMENTS table!!!
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.