The problem with him was not the speed, unless Malgus managed to use one sword at the crazy blinding speed of Grievous with four -

But the strength behind the attack was an issue. Because Obi-Wan was more than a foot shorter and at best half the man's weight. He could and did use the force to augment but every time their blades met it jarred his arms clear to the elbow.

So he adapted - more sliding Malgus' blade away with his own than direct blocking, more dodging and, yeah, a lot more work in staying away from him. He did not, however, allow himself the easy backing he'd been using. At least not immediately - he still retreated, but on his terms.

"And that mistake was?"