Things to check
IMHO, two things that can commonly slow down current scanners (many of them) if they occur at once:
Check to see if the four 'slow' channels are set DCS/DPL or CT/Tone squelch. Then check to see if your squelch setting isn't a bit too low. Sometimes scanners will check to see if the correct tone/code is received when squelch is set low/open.
Try turning the squelch up alot and see if that speeds things up. If it does problem solved, and just fine tune the squelch a bit after that.