I am using 18.104.22.168. I ran a 100 page job through a pre-flight node checking crop dimensions in under 2 seconds. What SW level are you using? Is it just pre-flight that is causing a slowdown?
The SW level can be obtained by selecting the "About" selection on the bottom on the GUI display. Within the blue splash screen a Version number is displayed (e.g., 22.214.171.124).
There are no pre-flight performance problems known within the 5.1.0.x releases. You could upgrade to the latest release 126.96.36.199. You could also contact Xerox support to look into the problem.