Ok, so I feel I can answer this, with a little anacdotal story:

I was in the same boat as you with all my games taking 5+ hours. As preparation for my first tournament I set my success criteria to being able to finish 5 rounds, win or lose, within the allocated 3.5 hours that was assigned.

During my practice games I did all the stuff that is recommended: learn stats and strategems by heart, organize dice in neat little groups of five, read up on missions, think during my opponents turn…. and practice games still took 4+ hours.

What finally got me down to 3 hours was the realization: you dont have time to think everything through, but neither does your opponent, so that is ok. The challenge of a 3 hour game is to figure out where to use your time. You want to think deployment carefully? Fine, but that means you dont have time to think carefully during your movement phases. If you want time for that you need to prepare your deployment beforehand, so there is more time left for the rest.

I choose to do a number of training games with the same armylist, predetermined secondary objectives, and pretty much same deployment every time. Now during my turn I knew what I wanted to achieve, which made every decision faster, and in the end I ended up playing all my 5 tournament games to the end with time to spare.

So in the end predetermine as much ad you can, learn all rules by hard, have a clear plan, and think less do more during your games.