Is this with a new project?
VDMX -> New Project.
Launch Simple Server
VDMX -> Media Bin -> Control Click -> Add From Built in Sources -> Syphon -> Simple Server.
Define slow down. How are you measuring no slowdown.
What is your monitor setup? Are you driving a ton of displays?
In other words, PLEASE READ THIS : https://www.mikeash.com/getting_answers.html