"Maintain High speed" could mean:
"there is no one in front of you, but there's someone behind you catching up, please accelerate at whatever speed you want ("just avoid supersonic please")
In that case he or she should specify a min speed for the one in front and a max speed for the one behind.
I believe another reason ATC would issue a "maintain high speed" clearance during descent is to word -simply- that the speed restriction on a STAR (SLP/speed limit point) does not apply? (instead of using the phraseology "no speed restriction").
If that is the case then standard phraseology "no speed restriction" should be used. Its understood by all unlike "maintain high speed".
Or perhaps it is because Controllers would know that most jet airliners descent at around 280 knots (if on profile), and that "high speed" would mean somewhere between 300 and 330 kts on transition, and that which ever speed we use within that range would suit the request?
That might be true but one pilots high speed is anothers normal speed.
The phrase "maintain high speed" is rubbish phraseology that unfortunately a number of my controller brethren use. 99 times out of 100 the pilot asks "just confirm what speed you want?" and so the controller uses up double the amount of RT time probably when he/she most needs it.
If you need to say it then you need to specify a speed, so why not give one?