Voyager’s QS drive definitely moved at the speed of the plot, from memory there was only a few seconds of difference in the amount of time they spent in the slipstream in the original timeline where they crashed, and the altered timeline where Harry’s calculations saved them. Yet that appeared to make all the difference between being on the edge of the Alpha Quadrant and still being at least 20,000 light years deep in the Delta Quadrant (going by the later jump in Dark Frontier).
Technically the TOS Enterprise must have had a superior warp drive to Voyager’s given how it was able to zip around the galaxy. Again, speed of the plot.
If we go by the dialogue from 'Timeless', the phase variance appeared 17 seconds into the flight.
That means that if we go by what happened the first time around, present day Harry sent the phase corrections which stabilized VOY for about 5 to 10 seconds, then phase variance started increasing and the ship was eventually fircefully knocked out of Slipstream (with an overload in the quantum matrix) roughly 2 minutes into the flight.
When future Harry sent the phase corrections which dispersed the Slipstream, the ship remained at flight shortly before it was safely pushed out of it.
So, there ARE discrepancies in how long each version of the flight lasted in the episode itself The original flight which destroyed VOY resulted in the ship being pushed about 15 000 - 20 000 Ly's to the edge of the AQ and lasted roughly 2 mins (it couldn't have been longer than that because we were told phase variance appears 17 seconds into the flight), whereas the altered flight had to be shorter. Based off what we witnessed and the dialogue, I estimated the speed of V2 Quantum Slipstream (from Timeliness) topped out at about 10 000 Ly's per minute.
Contrast this to when the crew first encountered Arturis and the fake Dauntless, it was mentioned that the ship could traverse 300 Ly's in 1 hour (which at the time was maximum speed - for sustained long term flight, the crew would had to have spent about 3 months in transit onboard the Dauntless to reach Earth if the fake message estimates were accurate - this suggests that even the fake Dauntless probably couldn't maintain maximum speed of 300 LY's per hour for more than 12-24 hrs).
As for TOS ENT having superior Warp drive... if it did, then what the heck happened to it?