My 5yo had his tongue tie snipped at 4.5 after speech therapist stated his poor pronunciation was totally due to his inability to use his tongue correctly - He had speech delay which we had thought was because of the tongue tie, but turns out may have been coincidental as he has other delays too.
Yes his speech improved afterward and with therapy was able to correctly pronounce all those letters he previously could not - joined sounds were the hardest to crack and it took quite a while for the old way of saying it to disappear...he sometimes got confused which words he had been pronouncing right and sometimes assumed all the time he said a particular letter sound, it was wrong....like his pronunciation of yellow was yeyyo....so he changed it to lello for a while...had to get him properly listening to hear the difference, which was a challenge in itself.
He can pronounce all sounds perfectly well now, if we can only get him to slow down and bother to actually pronounce them.......
..but then everything eslde is done at breakneck speed so why wouldn't speech also? That is another issue though!