Do any other game functions (scoring) seem to be moving slowly? A slow running game can be caused by the display interrupt generator running too fast on the MPU board. Pin 3 of U12 should read about 320 Hz. If it's running too fast, it causes the display interrupts to occupy a large chunk of the CPU time, slowing down the regular game functions.

I've seen boards with C16 missing, or one leg is broken. This can cause the 555 timer to run way too fast, causing the symptom described above. Note that if the game is running slow, the sounds will NOT also slow down (like a tape at the wrong speed), because the sound board has own clock and oscillator.