A few months ago My F150 was experiencing long cranks before finally starting. About this same time the battery died. I replaced the battery and the long crank issue went away. Interesting I thought at the time. I also began using the A/C all the time. When at an idle, the fan speed would slow then speed up as I accelerated. Well this past few weeks the long crank issue returned. My first thought was to check the battery voltage off and with engine running. Here's what I found:

Parked at rest, 12.56v
Turned the key on(a/c was on) and voltage dropped to 12.2v
Cranking it dropped to 10.6v then 12.6v at idle
Rev the engine it raises to 14v
Turned the headlights on with a/c at idle it dropped to 12.14v
Rev the engine it raises to 14v
Shut off it returned to 12.56v

So this morning I put the battery on a charger for 4+ hours. When I started it to come to work it fired right up. No delay. After sitting about an hour the battery voltage was 13.6v. So I obviously think the alternator is bad and order a 130a to replace the 90a that's on there.
Is it possible that this voltage issue could cause the long cranks? Sure seems like it.

 

Yeah, bad alternator.

Get a new one in there and verify it works properly, then see if the weird stuff goes away.

 

The lower voltage results in the starter turning slower so I'd say yes it can cause longer crank times.

 

Use your voltmeter correctly to test the charge system.
After cranking and starting the voltage should rise to near 15 volts then slowly drop back as the battery regains it's charge.
The charge time is determined by the health of the battery and how far discharged it is.
14 volts as a max after cranking and running some time won't cut it.
The battery is continually under charged.
Seems the regulator on the Alternator would be faulty.
Good luck.

 

Installed a new 130a alternator and all is well. 14.5V at idle with all accessories on. No more fan speed slowing. Starts almost immediately.

 

Good, I'm glad it was as simple as that and not some mystery wiring or regulator issue.