At the moment, I'm slicing up a clip, adjusting the playback speed for each the clip repeating that until I go from 1.00x to 3.00x (OK, it's motorcycles on meth, but I like it). Each step is initially the same number of frames. The change isn't linear because each speeded up clip is shorter than the one before it. I'm too lazy to do the math to get the number of speeded up frames come out to the original duration (30 frames at 1.5x becomes 15 frames duration - the clip has to be doubled to equal the 1x 30 frames). Naturally, the audio playback needs to keep the same speed(s). A filter with a starting speed, duration of change, and final speed will work. Adding one or more curves to the rate change would be nice. But a linear change, from start to finish, will be just fine.