All the effects in the Helix are modeled with the utmost accuracy. Some of the time based effects have limitations on the hardware when it comes to delay time or modulation speed.
If a tempo is tapped that is beyond the range of the effect model, the Helix will automatically choose the closest value that is in tempo and within range. For instance the Bucket Brigade delay has a max time of 300 ms. If you tap at 400ms (about 150 bpm at 1/4 note), a 200ms delay time will automatically be set.
So if it seems that an effect's delay time or modulation speed is slowing down as you increase the note rate, it's because you're beyond the limitations of the effect and the speed is getting cut in half.
Edited by Line6Will