When it comes to loading for different speeds, they seem to come with a certain "rhythm". My 4, 3.5, and 3 second loads all have their own rhythm to them. I try to take on my 3.5 rhythm in matches, because my 3 second rhythm feels on the edge of controllability. It doesn't have so much to do with the actual time of it as it does with what's comfortable, but still competitive.
"How fast you load will be determined by how much practice you've done before the match." This is very true, and I just found out it could be a bad idea to practice a lot before a match. I have a match on Saturday and practiced a bit too much last night, now my thumb hurts a very decent amount when trying to shove shells in at speed. Hopefully it gets better before Saturday, but I very well could've done this same thing a night before the match and it would've had a huge impact on my performance.