Any Known issues relative to performance??

There isn't any specific performance issues that I am aware of as we do have a large number of big customers who have users ranging in the tens of thousands range.

 However, with that being said.  K2 is a platform for you to build workflow applications.  So it is largely dependant on the design of the system (both from hardware and software perspective).  There are good designs and bad designs.

 e.g. If you are building an InfoPath application where you allow users to store large attachments in the InfoPath form (e.g. 2MB attachements), this wouldn't scale very well under load.

 If you have a customer account, you can log onto the KB site (http://kb.k2workflow.com) and look for the best practices document.  This is pretty useful document which details best practices (one of them is performance).