I built mrxvt from source in Cygwin several days ago, and have been using it for the last couple.
The need that I am attempting to solve with mrxvt is, basically, session management. Due to my work, I often find myself needing to maintain an ssh connection to 5 or more servers simultaneously, and mrxvt's tabs allow me to do so much more efficiently than having multiple xterm windows scattered around.
In the two days or so that I've been using the program, I have twice experienced a situation where mrxvt will begin using 50+% of the CPU of the machine, with a corresponding jump in the XMing window server. Each time this has occurred I have had between 5 and 7 tabs running, each of them with an ssh session to a remote machine.
When this begins to happen, the various tabs will appear blank. The consoles are still connected, but the display from each is not updated, and the already-displayed information is no longer visible. Scrolling up or down through the buffer does not work (or, at least, I can't tell if it's working because that's masked as well).
I can still switch from tab to tab.
On occasion, one tab will display the console (though I have not found a pattern to when it does so). When this happens, it is possible to interact with the session, but the response time of the window is very slow -- often there is a lag of several seconds before a typed character appears on the screen.
I have attempted to resolve the issue by closing some tabs, but without success. As far as I can tell, the only solution is to close mrxvt entirely, and relaunch it.
I'd be happy to provide any further information, debugging help, etc -- but some guidance as to what would be useful would be great.