lets think about this question for a minute.
ok, times up.
what would you think if I asked 'why do my tires wear out the middle of the tread first?' First thought would be too much air right? But what I didn't mention was I have 10" wide tires mounted to 4" wide rims. Totally changed the answer .
now to you question, I will give you a couple examples based only on your question:
Low battery voltage
Bad wires or loose connectuons
low fuel pump pressure
ok that's four, there likely are more but hopefully you get my point.
Here is a place to start your slow crank diagnoses:
Cranks OK, but No Start Checklist for Fuel Injected 5.0 Mustangs model years 1986-1995 A word about this checklist before you start: it is arranged in a specific order to put the most likely failure items first. That will save you time, energy and money. Start at the top of the list and work...
www.stangnet.com
'but I don't have a no start situation you say? This will cover it, I guarantee, jrichker and others have put this together and I will tell you it works, unless you have 15 to 1 compression and using a lawnmower battery to crank it.