I have a Craftsman L/T 2000 Mower with the B/S 18hp Intek. The engine runs smooth at all throttle positions including full when it is just running. When you start to cut 5" to 6" inch grass the engine seems to loose power. It still runs smooth but full throttle is only at about half, and the mower speed in gear 4 is like being in gear 2. In higher gear speed will bogg the engine down like cutting really high grass. I have replaced spark plug, fuel filter and air filter and pre-filter. Also sprayed some carb cleaner into it. With no noticeable changes. I have also pulled the mower deck off to check that all pullies are turning freely.

Any help appreciated!

Could be the carb needs cleaning, the valves need setting, or loss of compression (which I doubt).
Now if you take a small bite, you chew fine, right. If you for some reason were to fill your mouth completely full, then try to chew, you too would choke down and not perform very well. This is probably what is going on. Make sure your blades are sharp.