I'd like to give the Devs a big kick ass kudos. The performance in HF6 really rocks.
With a ~40 minute MP4 file with AVC video and AAC(!) audio. Timeline clip splits are basically instant. Timeline zooms get similar performance. This can be characterized as an effectively 'infinite' speed boost given the previous performance.
First import takes time to process/analyze the audio but that is normal. Especially for compressed audio like AAC or MP3.
Don't get many of those boosts, if ever.