Has it always been slow?
Is this before you open a project?
Connection to the internet? (hardwired, wireless )
If this is while a project is being opened
Nature and location of the data? (locally saved vs cloud etc)
How many map and layouts?
Lots of graphics?
Cleaned out your cache?
The list goes on
Do you have network connections set up? These could be broken and are taking a while to time out. Try renaming the Normal.mxt file (located in C:\Users\*YOURUSERNAME* \AppData\Roaming\Esri\Desktop*XX.X*\ArcMap\Templates) to see if that makes any difference.
I found a few resources to try with the slow ArcMap start up times.
Hope this helps! Let us know what you discover!