It's strictly a matter of how the bow stores energy. Given similar cam design, an 85%let-off doesn't store as much energy as a 65% let-off, which doesn't store as much as a 50% let-off. Less stored energy available, the less energy can be transferred to the arrow, so the less speed the arrow can have.

BUT - A great deal depends on how rapidly the bow drops from peak draw into the valley. If it's a bow that maintains peak draw weight forever, until your eyes bug out from the strain, thensuddenly dumps you like a gob of mud into a narrow ditch, the difference in performance will be negligable. Stored energy will be very close to the same. If it's a long, slow let-off that cradles you gently into a comfortable, wide valley, then the high let-off bow won't store nearly as much energy andyou'll see a greater loss of performance with increased let-off.