Nvidia advertised the GTX 970 as having 4GB of vram running at 224GB/S.
HOWEVER, it has been revealed that the first 3.5GB operates at 192GB/s (7/8th) while the remaining 0.5GB operates at 1/8th that speed. A quite a drop in ram speed and this does effect performance.
When I ran out benchmark tests on both systems, we were working with HD footage only. I must say I didn’t notice any problems. I was monitoring the video ram usage and it never got over 3GB, so I wouldn’t have had any problems.
While gamers have reported problems with game playing, I have heard from a couple of video editors that are having problems with 4K footage. They are reporting that when the video ram usage goes over 3.5GB, they have noticed Premiere starts to slow down at times when it is rendering, exporting or during playback. William Beck, from London wrote me and said if he switched his video card back the GTX-680 he didn’t have this problem of slowing down.
Now other people who are using 4K media with the GTX-970 are not having any problems. This is most likely due to the complexity of the time line.
You should be aware that using 4K+ media that really starts to push over the 3.5GB video ram limitation.