Not really a drill but emphasizing two cues helps with off speed.
- "Both feet on the ground at launch" will help prevent unweighting of right foot at start of swing....improve lower half timing for both soft and hard slaps.
- "Pull back with rear hand at start of cross" will help prevent front side from opening prematurely on hard slaps.
Also, sometimes helps to focus having on a short cross step...especially for hard slap. Seems easier to maintain timing when distance travelled is a little shorter.