I haven't seen anything that put Kessler below Wesker in speed. The only difference is that Kessler teleports while Wesker just moves his legs. They both have insane reactiontime and Kessler can easily keep up with Wesker when he makes a move.
Kessler has the power, but Wesker has the strength. Wesker is stronger as well as more durable, but his most significant advantage in any battle will not allow him to beat Kessler here (His speed).
Which is why I think it's an even battle.
Wesker has higher durability and strength. Because the fight takes place in a city, the possible speed advantage can be leveled thanks to corners and narrow passages. Kessler needs time to dish out his strongest moves, which is time he doesn't have in this blitz battle.
(Although a single Lightning Strike is enough to take Wesker out, so he best not stand still much)