There always seemed to be at least a 2 hour long line at the US counters in PHL. You should be able to access the sterile area in civvies with only your badge - just no toiletries allowed. Also all the terminals are connected inside security, except F - the express terminal - which has connecting bus service ramp side. (Or they were my last trip there - its been a while and there was some reconstruction.) So enter at the terminal DL uses to help ease any TSA concerns.
Of course on any given day PHL can out-delay LGA, so if timing is a factor in your consideration, you might look at your other options.