I have yet to be issued a crossing time for a waypoint where I had to speed up!!!
At least once a week, we will get a response from a pilot that he can't get there til 2 or 3 mins after the time given. This is even when using a prayer wheel and allowing for slow up on descent shows that the time given is a slowup. To be diplomatic, I say cross not before the original time and funnily enough they always cross at that time - must be an FMS issue.