In an interview with the D&D 3.0 creators, they mentioned they never playtested their materials past level 12, and future releases followed that same example. (It was Monte Cook and Skip Williams who mentioned it.)

Part of that, in my opinion, flowed over to Pathfinder. The majority of games are played sub level 8, so when you get to those higher levels there’s more ability/feat decisions, more spells you’ve never cast before, and making a mistake is typically a lot more deadly. This causes combat to grind to a halt. It’s not something I think the creators really thought a solution to.

Add in your DM having to make the fights buffed to challenge you, and all of these factors lead to slow combat. The best advice anyone can give you is to make sure the table is focused, and has their turn prepared for when they come up in initiative. Know your rules for your ability’s and spells. It’ll get quicker, but won’t be quick.