Hey Liam! Welcome to the forum!
Generally speaking, once you can play a song without looking at tab, it’s OK to move on. That being said, it’s important to try to identify WHY you’re not able to get it up to speed. Sometimes, folks accidentally use the wrong picking fingers, or have some other bad form in either hand that may be slowing them down.
Keep in mind you have to allow yourself plenty of time. I don’t think of myself as REALLY knowing a song unless I’ve played it through at least 1000 times. For example, I recently learned Beaumont Rag. Now that I have it up to speed, I figure I can play it though a little less than 100 times per hour. So until I have at least 10 hours of playing nothing but that song, I don’t feel I really KNOW it. Others have different methods for learning a song. That’s just what works for me.
If you have some time, post a video of yourself playing something you can’t get up to speed and we’ll try to see if there’s something we can do to help.