I have dual monitor setup on my HD4870 and wanted to hook up my HDTV so I added an X1300. I noticed a slowdown anytime there's anything on the HDTV: video playback is very choppy on it and slows the whole system down. The CPU utilization doesn't go any higher so it seems there's an issue with hardware acceleration. Even if I move a video window partly into the 3rd monitor/HDTV where the X1300 can take over the playback, it makes other animation like moving windows around on my other 2 monitors slow. I updated to the latest Catalyst from 12/10/08. Could the X1300 be a faulty card?
I once had a 2 monitor setup (on a single video card, first with an x800pro then a 8800gt) and what would happen, for example, is that if I switched my MPClassic from one screen to the other (after a file had been opened), it would either go black or be very choppy. The solution was to start the program, move it to the monitor I wanted to use and then open the media file.
This behavior would be different depending on the drawing mode selected (dx7 or 9, windowed or not, etc...) in the option menu. I suggest looking into it and trying different options until you find satisfaction.
I tried all the different rendering modes but the default was fastest. It's definitely a weird driver problem because as soon as I made the HDTV the primary display it was fine, but as long as the primary display is on the 4870, playback on the X1300 is slow. This is the same the other way around: if the X1300 is the primary display then playback on the 4870 is slow.
I'm surprised this isn't mentioned more often. Anyone have any ideas?