A gentleman from the UK emailed me about a problem he was having with the slow motion feature in Adobe Premiere CS5.5 when using the Mercury Playback Engine in the GPU mode. The slow motion was not smooth, it was jerky. When he set the MPE to software mode, the slow motion was great. After he sent me a copy of the clip, I ran a large series of tests using several different video cards, so we could rule out the video card was the problem. No matter what video card I used, I got the same results as he did. (Note: Frame Blend was turned on)
When setting the clip to a slow motion speed of 50%, it looked a little better but, it was still not smooth. When the clip was set to a slow motion speed to 30% it was really noticeable. With the clip set to 15%, the problem really stood out.
Here is a video of what I am talking about.
I spent a week testing this problem on 4 different computer, with several different clips in a variety of video formats and exported them to a variety of output formats, all with frame blend on the timeline on. All of the clips show a varying degree of jerking or stuttering when in the GPU mode. When I tested them in the software mode, they were all smooth.
If you are using the slow motion feature, you may want to watch out for this problem. If you are running Premiere CS5, you will be fine in the GPU mode with the slow motion. Hopefully, Adobe will adress this with an update, as this is a problem for those who use slow motion.