Yeah a PLC isnt going to make a ride start or stop faster, the main reason for having two PLCs is so one of them can be a watchdog to the other.
Ibelieve what the Phantom Flyers recieved new was a squirrel cage motor, which is more reliable than an induction motor.
Technically I guess a new PLC system could casue the ride to start and stop faster, but it really has nothing to do with the PLC itself, it jsut has to do with the timers and such. I don't know how the old system was set up, but maybe the new PLC if there even is a new PLC controls the Variable Frequency Drives differently and has a shorter accel / decel parameter time then before, or maybe they are using injection braking to stop it quicker, instead of the previous ramp down type at PKI.