Leveling up on the Sunday morning took a chunk of time but I am not too worried about that as in my next campaign I will be playing a level-less game so that will not happen.
I am working on things that can improve my prep time now.
For me, the PC creation process is the most time consuming. I would try to get the players created prior to game day.
Leveling up wasn't so bad but I made the players do some housekeeping at time of creation. The players tended to know what skills they wanted to purchase as they play through the sessions. I made character sheets with a space to put the skill cost so they wouldn't have to flip through books and there were blank lines to add skills and costs that they want for next level. At time of creation they would be prepping for level 2.
1/4 Spear ****000000 ST/ST/AG
1/5 Riding **00000000 AG/ST
2/4 Iaijitsu 0000000000 AG/QU
* - I'm just pulling stats out of the air for examples
When this PC hits level 2, he knows the costs per rank, and the stats associated with it. If the stats changed on the stat gain roll, then he knows to recalculate those skills. That takes a lot of burden off the GM at level up and the player knows ahead of time that Iaijitsu is a skill he wants and the costs and stats needed. No need to flip through books to find the info.
Also, when players leveled up, we tended to break for lunch/dinner. During the break in gaming, the players would tend to their PCs to not take time away from actual play time. Some would play as they were leveling up because they didn't want to stop game flow.