in bursting, it has nothing to do with spd, just level.

add all the levels of your party (including pets) then divide that number by the number of people in the party (including pets), subtract that by 25, that will equal the level of the mob you can burst on with 100% chance to combo. so for a 100% chance to combo on a lvl 10 chicken using lvl 1 cows, the players in the party have to be an average of lvl 69.

Example: (lvl 69 character x 4 = 276) + (lvl 1 cow x 4 = 4) = 280
280 divided by 8 = 35
35 - 25 = 10
lvl 10 is the highest lvl mob you can fight for 100% combo attack

if you are training on something near your level say with pet training, you would want to get your speeds to match or be as close as possible for the best results in comboing