A bit of research on the strings I use (D'Addario ProSteels) shows that the individual string tensions for 5-string long-scale set is a little less than 20% higher (on average) than the 4-string medium scale set.
 
Interesting.  Taken together with the scale-length differences, that would logically account for what I felt, speedwise.

String set up can also affect speed of play as can the neck style. When I first got my MK 5 a few monhts ago I was amazed of the speed.  
It was setup totaly different from my Rouge 5. Both are 34 scale.
 
(Message edited by thumbsup on March 17, 2011)

Depending on if/how you anchor your plucking hand, a different bass can really affect that aspect of your playing as well. When I play the five, I rarely use the B string as anything other than an anchor. I might use actual notes there on one song in five, but it provides a nice anchor spot. For obvious reasons, it's a different place than where I anchor while playing a four...
 
I would also suspect that playing across a variety of instruments on a regular basis would make you less dependent on a particular scale or spacing. If you put a billion hours in on a shorty, though, it might be less comfortable to play a 34 or 35 inch neck. I know I wouldn't want to play a long scale fretless after the SC. I can feel the cringes from anyone out there with a great sense of pitch just thinking about it.
 
-bob