Twixtor will work but it won't be in real-time for HD size frames. It involves render of that section. The more CPU you have the faster. Aside CPU, disk IO bandwidth and the nature of the media affects the render time. For example although so-called camera native formats can be played in a streaming manner fast, they achieve size compression by using what is called long-GOP encoding (e.g. a group of 15 frames is encoded together). So since Twixtor collects two frames on the timeline to make one it ends up some apps (twixtor is a plugin) ends up having to retrieve 15 frames at once to read a frame... so why for example it's much faster from ProRes for example, or in Avid the equivalent intermediate codec they ingest the media to. I think for example that's why FCP marketing demo at NAB put so much emphasis on editing native media as they are one of the apps that could be better in that area.