Hey Chip, a bit of a guess
, I remember a time (and it's from the era of the SPX90)when you sometimes had to program some midi latency to get certain devices to play (behave) with each other (this is back in the Atari sequencing days using Quebase or Notator and Alesis drum machines and Korg MOne Keyboards etc. recording analog was still using tape with a SMPTE track on the tape to lock them together). Now I have an original SPX90 and never midi'd it up, just changed programs on the front. However, yours is a later variant and "may" (pure guess here BTW) have midi latency options in the menu some where (worth a look). Otherwise, it should change ~instantly given you are feeding it from midi through (unless the MP-1 has latency options for the midi through which I don't recall it does).
Then again, it may just be that gadgets from that era didn't react as fast ?? but half a second is allot. That said, the original MP-2 had quite a lag between program changes which ADA fixed with a firmware upgrade.
It may also be program dependent, i.e. if you change from a big reverb/delay whatever, the SPX may delay the change until the tail of the signal/effect is finished so you don't get a glitch (smoothness)??