Problem with Custom Property Tab

I have a customer that has a problem when they have the Custom Property Tab open in an assembly and they try to ctrl-select multiple parts in the graphics window. The 3rd part they select takes a long time to process. Any parts past 2 take much longer for each new part.

They used the Property Tab Builder to build the interface to add properties to assemblies. I was not able to reproduce on my computer, but they are using SW10, sp 2.1. Has anyone else seen this?

I have seen the SPR 440006 in the knowledge based that says the Custom Property Tab affects SW graphics, but that's pretty general.

SolidworksGeneral