I agree with Snips. I was scratching my head on why you think it is running slower. Snips is the Guru so I revert to him. Anything I say is just tossing thoughts out there but there are two factors that might make the speed look slow; the transmission ratio and advance/ scrolling speed. The transmission ratio is varied by the range, Noise Limiter, PRR level and TX Pulse. You might try to set Noise Limiter to 0 at NL menu, PRR Level should be 20 and TX pulse should be shorter than standard(STD) under TX/RX menu. Keep in mind if you are losing the bottom echo (like due to turbulence), that will also makes the transmission ratio slow for searching the bottom, so try Bottom Search Mode OFF under E/S menu. (and maybe fix the cause of the turbulence if possible) The advance/ scrolling speed is changed by the advance speed at PIC advance menu. Scroll time and smoothing settings under DISP menu.
The picture advance should normally be 1/1, and scroll time and smoothing settings should be OFF.