From your description I'm not sure if it's turning over slowly or if it's turning over a long time before it fires.
Right after I got mine, It would start right up when cold, but after it warmed up it would crank & crank for 15-20 seconds before it would finally fire.
Took it to the Isuzu shop & they replaced the fuel pressure regulator (they indicated that this wasn't the first time they had seen this prob.)
She's been running great ever since. Hope any of this helps.

Ldub