If the VCO have to be tuned in big frequency jumps it will take a longer time to do so. If it only matters the VHF marine band within a 5MHz range it doesn't affect speed at all.
If you have say 137MHz frequencies alternating with 175MHz ones it would probably be seen to slow down scan rate. It's always a filtering capacitor to the VCO steering voltage to get rid of the PLL step noise, that sounds like normal weak signal hiss, and that capacitor takes time to charge or discharge when the frequency has to change. Old synthesis scanners that did 100ch/s scanning often had a small capacitor, or none at all, and had an incredible high background noise level even when receiving strong signals. That high noise also made the squelch think that the signal was weak and had trouble stopping on carriers, so now scan speed are reduced in modern scanners when bigger capacitors are used.
Professional radios have a slow scan speed but then also use bigger capacitors that totally eliminates any hiss from the PLL noise and produce a clean audio like old times crystal scanners that didn't use a PLL. Some advanced radios use a small capacitor while scanning and adds a second big capacitor in parallel when scanning stops.
/Ubbe
