Maybe of use to some - especially Rasp Buster/Stretch - but on an old RPI3 I had slow boots and reboots for REALVNC server connections.

However it seems its not the vnc server but a headless problems - no mouse seems to be the problems reading the thread below. Any how installing HAVEGED cures this problem and now vnc sessions (especially on reboot) and faster (alot). Dont know if it works on other O/S on RPI.

www.raspberrypi.org/forums/viewtopic.php?f=28&t=230779

linux.die.net/man/8/haveged

If it helps someone great.