Stream clip is super easy..
Keep in mind its not always the actual pixel size of the video that slows a machine (1920x1080) or (1280x720) but the data rate..
the higher the data rate, the harder the cpu has to work to play the clip at its native frame rate..
Also, when you lower data rate, but retain the pixel size (1920x1080 in this case) the overall picture will degrade dramtically.. but if you lower pixel size along with data rate, it will look better..
so when you knock it down to 1280x720, drop the date rate to around 5000 kbps and try that..
Codec is also a factor.. which is how the file is made... try an h.264 codec as an .mp4 file..