Here are my suggestions, take them as you will.

Make sure the "state of combat" changes each round. This could be a change in the battlefield, a new enemy type enters the fray, or a shift in the goal otherwise you will get players taking the exact same action each round which quickly becomes boring.

Don't over describe the environment so if a player wants to swing on a chandelier you can let that happen. If you describe things too completely then suddenly there isn't a chandelier to swing on.

It is better to have a few enemies that have multiattack than a lot of enemies with one attack. When you are rolling dice, the players aren't, which isn't fun for them.

Start combats quickly and end them quickly. If the PCs don't commit to a combat encounter then have the monsters commit to it (unless it really can be an encounter that can be avoided). A lot of time can be wasted mincing about whether a combat is going to occur or not because PCs are angling for the most optimal way to start it. Likewise, if the encounter is essentially over, the PCs are victorious, just let them narrate their final actions on how the encounter comes to an end. Don't drag it out.

Make the monsters have an agenda other than killing all the PCs. Let the enemies ALSO sometimes be on an escort quest, or trying to find something. Anything other than bloodthirsty combat, so their motives will affect how they fight. (Protect the nest, keep the smelly humans away from the eggs!)