Two questions
 

How come it seems like it takes a football field to turn my 1000 and why when I'm in first gear wide open throttle and I get into tall thicker grass it seems to really slow things down almost like it's not running enough rpms. Normal grass grow its perfect but that thicker stuff kills it

Will it turn shorter one way than the other? Is the toe in set correctly?

Does the engine reach 3600 RPM? Is the governor spring in the correct position?

Also, tall thick grass will slow anything down with a mower deck on it.

Check your toe in like Jonathan said it may need adjusting. There could be several reasons it slows down are your blades good and sharp? Just how tall and thick is the grass? Is your engine worn? How big is your deck on your cub? Is the timing correct on the engine?

How big is the mower deck?
The wider the mower is ......the more horse power it will need.

Do you still have the orginal tires on your 1000?

As to the slowing down, it is a little unclear if the engine actually pulls down or if just the blades slow down. If it is just the blades slow down, I would check the belt system for wear and proper belt tension. Check the clutch as well. If the clutch is slipping, it will get really hot when that happens. Otherwise, there may be some adjustment off, like others here have said.

It'll turn a lot better to the left than the right and the clutch doesn't slip it's like the engine isn't turning fast enough for it. It's a 44a deck

Does the engine use any oil? Could be worn out my 1200 would do fine until you got to thicker grass then pull down. The engine in it is sitting on my bench awaiting overhauling . Could be the carb is out of adjustment to.

Hmmmmmm...

I seem to very vaguely recall that IH offered a "blade speed improvement kit" for the 44a & 50a. Not sure of the details, but ( I think ) there was a different center pulley for it.

Yeah, it was the speed up pulley.