Never seen Sillakh but yeah, both Pascal and Dawson are quicker then Cleverly. The thing that impresses me about Cleverly's hand speed though is the way he looks a bit awkward and lanky and doesn't seem like he'd be fast at all.
I would say Cleverly has about the same hand speed as Dawson and Sillakh, while Pascal is the fastest in lhw. But Pascal trains very much for speed and to be explosive. That's why he fights in bursts and seem to have some stamina problems.
cleverly puts punches together in combinations better than any fighter in the light heavyweight division. his punch output would totally expose chad dawson again. however, i think cleverly needs a couple more fights and his defense tightenning up before he takes on any of the beltholders.