120fps 720p footage with real time playback on Ubuntu? + editing?
While it is true that 720p video only has about half the data of a 1080p feed, the other thing you have to realize is that when you push the system beyond it's limit, it may spend a lot of time trying to process frames that it doesn't finish in time.  Depending on how the player is configured, it may give up and try to catch up rather than finish rendering the frame and get further behind.

Because of this, as soon as you get behind, you may now end up finding it is much harder for it to catch up.  You also run in to problems with keeping the data flowing quickly enough since the level of compression sounds like it may be higher (harder to decode) and the CPU is so busy that it can't be working on moving data up to be ready to read.

You probably should do a transcode of the video to a lower framerate and work with the transcoded copy to make editing decisions and then swap in the high framerate video only at the end.  After all, the only difference 120fps video will make on final output is anywhere that you end up doing a slow motion effect since you aren't going to have any display (including your computer screen) that can actually load 120 frames per second in to the image buffer.  Even 240hz screens are not displaying that many frames from a source.