When ATC say "maintain high speed" or "give me best speed" how fast do they mean! High speed to me would be 320kts but maybe ATC have a different speed in mind.
It's an informal request and means speed up if you can, as much as you're comfortable with. We'll watch your groundspeed and issue something more specific if it's not enough. It avoids more radio transmissions and having to slow other aircraft.
Same with "maintain slowest practical speed" -- the slower you can do now, the less vectoring and speed restrictions you will probably get later.