What version of window 10 and what version is the vmfs is the datastore using vmfs 5 or 6. There was an issue in 1809 and vmfs 6 that had a slow startup but the latest updates to windows and esxi should have fixed that
Outside of this are you doing instant clones, full clones, or linked clones? Have you tried to watch the process by connecting to the vms, where is the slowest part that you can see. Do the vms themselves start immediatly but windows starts slowly or does the vms themselves take time to start,.