Well, back in the day IRC was born it was meaningful, as you would normally come out of a shared box with your own actual user account. Nowadays it’s pretty much pointless and most newer IRC daemons allow you to skip ident check entirely as to not waste resources on the server, but some big networks keep up with the tradition because a “properly configured client” (ie. one that provides an ident reply) is less likely to be a spambot. In fact, some networks have stricter I-lines for clients without ident.

No, unless the server has very little RAM or a very slow CPU it won’t affect the speed.