Hum, i have a decent(ish) pc for movie making yet Fraps is utterly slow for me and its lagging no matter what im recording, or even OBS cant handle recording properly for me. Not sure where the logic is.
When you right-click the replay file theres an option to export video frames. It has a downside though as its video only, no audio - you have to record it with something else, like Audacity. You can then import all these frames in Vegas and work with it, much faster and quality is really good.
This is how this method looks after render in Vegas, 1080p at 50fps. Note that Vegas sucks for Audio editing and it can be really tricky to manipulate the velocity if you want it to synchronize with video nicely. Now
this is 150% speed video at all times (audio recorded at 1.5 speed in WA and then video velocity changed to 150% in Vegas), if i was about to slow down here and there and leaving some parts at 100% speed it would tough.