I noticed certain trains on a certain branch (not mentioning specifics) enter valley stream at track speed considerably relative to most other trains that come to a crawl prior to entering the station.
Is this bc train is late, or some other reason, or is this my imagination?
Valley Stream Station is close to Valley Interlocking station,where trains headed to West Hempstead Far Rockaway,and Long Beach Branches leave the Alantic/Montauk(aka "Babylon") branch mainlines to their destinations. Speed will depend on signal/cab signal indications. Believe Valley Stream is considered part of the "Long Beach" branch service,with limited service by Babylon branch trains.
I think Backshophoss answered it. A more detailed answer would require knowing exactly where from and where to the train is going. For example, I would expect a Far Rockaway local to get a fairly restrictive indication coming in, whereas a Long Beach local goes straight (coming from the Atlantic Branch) and would likely get a Clear indication. For a westbound train from either LB or FR, a lot would depend on whether a Jamaica-bound train off the other branch had just left the station ahead of it. Hard to know without more details.
The curve to Far Rockaway branch is 15mph restriction. Trains routed that way approach the station at 15mph account of the way the signal department has the codes set up and the way the ASC system works in conjunction with the fixed signals. The 15 forces a positive compliance with the slow speed aspect at the east end of the station. Long Beach branch trains can pass No2 track at 45mph and 60mph on No1 track in the station. As for any other route it's a combination of the switches speed rating and ASC which determine the speed.
Long story short, Far Rockaway trains go painstakingly slow because of the curve off the main onto the branch. PTC will make commuters happy when it slows things down even more or so it's been told.