As a former JUCO head coach, I can only guess that you are speaking of distance events when talking about how slow they are at nationals.
Having been in the KJCCC of the NJCAA, I can attest to 3 reasons why the distance events are a little slower than what they are capable of being.
1) The NJCAA meet has been in Odessa last couple of years. Basically 100 degrees and no shade. Not great for a distance runner obviously.
2) Doubling and tripling, plus there's a 4x800 event that some might also run at nationals.
3) There's not a lot of depth in JUCO. Usually the top 3-4 in most events are DI calibar but after that it falls of quickly. So, a distance runner doesn't necessarily have to run his personal best to win. Sit and kick is the appropriate situation.
Yours in Track,
Mike Cunningham
Asst. Women's Track
Ball State University