You don’t need to spend anywhere near $5k to have a fast bike.
We did a writeup on 3 bikes under $3k the other day and all are pretty much no excuses bikes you could race all the way to Kona:
http://austintriathlonstore.blogspot.com/2014/06/three-tt-bikes-under-3k.html
Those are just three examples, Trek, Felt, Quintana Roo and others all have comprable bikes in that price range (or lower!)
The difference beetween a road bike and a TT bike is large. The difference between a medium-level TT bike and a top level TT bike is small. I wouldn’t advise someone to spend $5k+ on a TT bike unless they were both wealthy and top level time trial competitors or pro level triathletes.