Depending how it is used it is a perfecty acceptable, and indeed sensible instruction. Using the FL150 by TIGER restriction... quite (very) often there will be aircraft in trail all descending to FL150, all on speed restrictions to maintain separation.
There is 10-15 mins holding for EGLL. You are transferred to TC who need separation by the time you hit the hold.
You are number 1 of 3 in trail.
You call on "ABC123 descending FL150 level TIGER maintaining 300Kts".
Controller replies "ABC123 Descend FL130, when level reduce to holding speed, delay 10-15 minutes".
This instruction ensures you maintain the speed control set up by previous sector, and lets you slow down as soon as you have reached a level that leaves space for the following aircraft.
If the levels are available at the hold, it could just as easily be "descend FL100, reduce to holding speed on passing FL130, delay 10-15 mins"