If anyone's seeing rapid growth spurts on a program where you visit every 5 weeks you are applying the wrong blends of fertilizers at the wrong rates and theres no way to avoid a little extra growth in spring no matter what you use. With 20 to 30% slow I see no rapid growth issues of any magnitude on my lawns and no complaints from clients. Now back in the day when we were applying 1 lb of N per thousand using 46-0-0 dissolved in water we would see it. Nowadays 1/2 to .75 lbs of N is more mainstream and there are some great slow release product available from 20% all the way up to 100% slow. 30-50% slow is commonplace.