Mine did the same thing ( of course we all know the stock gauges don't mean a lot, lol) a simple way to check it is to jump it or swap a battery with another vehicle( this is how I go about it anyway), if it's still slow it could very well be the starter. The only reason I say starter is because mine acted like that, it would click, click, click until the starter finally turned cranking it over slowly. Tapping the starter would make it kick in sometimes too.
89 YJ mercedes turbo diesel/AX15,restored Ramsey RE 8000
14 Ram 2500 4x4 cummins 6 speed manual :)