When to use a rate instead of a shift
The video was sped up or slowed down as a whole (say 0.9x or 1.25x in the editing software) while every cue kept its original rhythm: the beginning matches, and the further you go the worse it gets — with a constant ratio. That is a rate problem. If the whole file is simply offset by a fixed amount without growing, a shift is enough — see moving the whole subtitle file earlier or later.
Which number to enter
Subtitles must follow the video, so enter the factor the video actually changed by: a 1.25x speed-up means cue times multiply by 1.25 to catch up; a 0.9x slow-down multiplies by 0.9. In the timing editor, add a Scale by rate step and put the factor in the value box.
How this differs from a shift
A shift moves every cue's times by the same fixed amount and keeps cue durations; a rate multiplies both start and end, so durations scale too. After a speed change the durations themselves have changed — that is exactly why a rate is required.
If you are not sure about the factor
When the factor is lost, fall back to two anchors: enter the real video times of the first and the last cue and everything in between spreads out proportionally — equivalent to solving for the right factor. See subtitles getting more and more out of sync.