Hercules' fighting speed.
In the "Hulk vs Hercules: When Titans Collide" one-shot Hercules was shown taking the Hulk down with an impact described as being "slammed into the side of a mountain by a jumbo jet at top speed." Immediately after this takedown, Hercules is shown on top of the Hulk raining dozens of unanswered blows at what is obviously a superhuman rate of speed. This battle and the time he fought the Sentry and was able to dodge his flying super-speed dive attack, and even grabbed the Sentry's cape on the way by, prove Hercules has some measure of super-speed and that he has used it in battle.
Why he isn't depicted using his speed in battle more often is probably because it would give him too much of an advantage over his opponents. That's my opinion why most Marvel writers/editors don't use or allow Herc's true speed to be used.