My first "fast mover" was a -10 powered Merlin III it was about 100 knots faster in cruise than anything I'd flown before and it was all single pilot. Cruise isn't what messes you up it's adjusting to the timing below 10K at 250 KIAS and transitioning to approaches happens a lot quicker but once you get properly ahead of the thing it boils down to basic flying skills and in particular basic attitude instrument flying. If you've got a good solid scan the rest of it all falls into place.
My first jet was a DC-8 and I found that thing relaxingly simple to fly after coming out of flying the Merlin single pilot. The work load was about half. During initial we had a guy who was flying DC-3's for Four Star down in Puerto Rico he was my sim partner until he busted out. His problem wasn't the speed his problem was that his instrument skills were horrible. He'd just spent WAY to much time flying VFR. Most guys who have trouble transitioning to jets have some basic instrument attitude flying issues. That's what I've noticed over the years anyway. Because a Jet takes a more aggressive scan simply because they do things faster and in particular the tiniest amount of pitch change equals much larger rates of vertical deviation than most straight winged TP and Recip guys are used to.
One thing I remembered the old timers telling me, the guys who had transitioned from large piston bird to jets at their airlines in the day was that the VSI was your life make sure you look at it twice as often as you used to. Pretty sage advice for a guy transitioning from "those" to "these".