DM's work is a hard work.
It's hard expecially because you have to do a lot of things.
A Golden Rule of DMing is
More things you do, more troubles for you, more mistakes by you.
So you must focus on important things and let others do the rest.
Who are others? Players, of course.
What can they do for you?
A lot of things, for example:
Drawing tactical maps
If you use a grid or if you want to show the area of an encounter to your players, let them draw it. Simply describe area's dimension (i.e. a 20' × 20' room) and what's inside (monsters, fornitures, etc...) and have them to place them on the grid or drawing on paper. At the end make only few changes if you need or if they were wrong with something.
Keeping track of monsters hp
Players at my table were used to track hp monsters for own purposes. So I let them the only trackers of monsters hp. I as to them from time to time "how many damage has taken that gnoll?" just to know how badly injured are.
Keeping track of time
Let them track time. Both calendar time ("how many days have passed?") both combat time: spell durations, feature durations (i.e. barbarian rage..) and so on.
Give them to players. They will have more and different things to do. Of course you can always modify their decisions, according to NPC's behaviours and motivations and by circumstances, simply saying "She refuses to do that" or "Instead of charging he runs away".
Don't ask anyone which number they rolled. Start counting down from 25, ad have you stopped by them everytime you reach one of their numbers.
Or let them track on paper.
Easier and faster.