I find the higher level encounters are a bit harder to prepare for, as the PCs can really do damage fast but can also get tagged fast too. I prep by beginning with what they are going to face, do a quick CR calc and write everything in a MSWord document. I paste over critters and NPCs from generators and tweek them to meet my needs, such as if I want all of the NPC humanoids to be good at tripping & throwing.
Later, while walking to and fro or sitting somewhere boring, I think about how I want a battle to progress and what the players might do. It never happens that way, but it helps to know that I should increase the AC on dire bats to foil an easy kill. With the higher levels, knowing the casters' spells becomes more important. Fireball & Magic Missle may be a cliche that makes you yawn but it works, and they will use it, and there are the similar standby spells in UA. Give the BBEG casters a shield spell to prolong the fight so you can get in some good licks on the party before losing.
Then I go back and make the last changes and lay out the landscape where it will take place, which goes quickly for me. Totaled, it probably takes me 2-5 hours making notes, maps, role-playing encounters and baddies to fight. And that's the fun part I enjoy away from the table. It's not that long, and if I am running out of time I can cut corners by skimping on details here and there.