I haven't flown into SXM so can't comment specifically on that airport's ATC operations or procedures, but in general the Caribbean operates with the same "no problems, mon" attitude that makes vacationing there so relaxing. Most airports there are basically single runway operations and "unhurried" arrival and departure sequencing is the norm.
There are areas without radar coverage where aircraft utilizing the same routings are spaced by time, and even where radar coverage is supposedly available local outages are not terribly uncommon (along with the relaxed pace of restoring service).
All that said, I agree with others that the 1.5 hours should be ample as long as the flight to PHL is close to on time.
Jim