we are having LR 4 issues sadly
so did a clean install of
SL
Lion and Mtn Lion
so had a chance to test LR performance on a clean install vs older and the newer was a touch quicker ! I mean a touch we are talking less than .x seconds and not a hard triple test just a quick timing test
mac pro 5,1 using SSD for each install etc..
I have tested LR 3 quite a bit in the past and did some testing on a older installvs new clean with all programs needed in place
with lots of apps and such and found a clean install was faster by a very small amount but it was quicker
I tend to test this a bit cause I like to
and seems each version the need for fresh is going away
now the thing is will that savings of .2 or less seconds per thing you are doing add up to save you time of installing everything from scratch ? if you are a business then I would say NOPE
if you just like to have fun and tweak then go for it and test and find out for yourself
again I think the time it takes to install a clean OS vs just updating you are actually behind in time !!!
but its fun
also like some said its about sometimes you have to sometimes just keeping it clean as you move along is best
delete a program make sure you yank the pref files and support folders