Yes, it does seem to be months before you can see an upwards trend on sync speed after having an issue with your line on fibre.
DLM normally starts correcting after around two weeks, and fully correct within a month. Still pretty slow, but ....
I suspect that if slower speeds go on for months, it is because the line remains in a bad state for months. It is not uncommon to have mysterious noise (often intermittent and virtually untrackable) for a long period, and then for it equally mysteriously to disappear.
DLM normally starts correcting after around two weeks, and fully correct within a month.
Not so ... line management runs 24/7, the return to MSR thresholds can be as little as hours if conditions permit ... and according to how long the shift away from MSR happened.
e.g. passive environment interference ... thunder storms.