My second child was tongue tied. It torn on it's own when he was maybe a year. It was painful, and the dr gave a script for a topical numbing gel.
My third was also tongue tied. We decided to clip it. It was a simple proceedure. They put a clamp on it, then cut it. It was a bit hard to watch, but he was fine shortly afterward. I was surprised that they weren't planning to use any pain medicine or numbing gel. I had brought the numbing gel that I had leftover just in case. When I mentioned using it, the dr was like "that's a good idea". Yeah, you think!
So if you have it done, ask them to call in a prescription for you ahead of time. Pick it up at the pharmacy and bring it with you.
If it's affecting the nursing, I would probably go ahead and get it done with. Just insist on the numbing gel, or at least get some good over the counter stuff.