Burn rate does not translate into Isp and vice versa. However BP with a higher burn rate could, for example, turn a baby F15 into a baby F25. Higher thrust but shorter burn time, giving essentially the same Isp and total impulse.
Back in The Land Of Ago, I attended Pyrotechnic Guild Int'l meetings. One activity was Pyro Golf. Participants made the fastest BP they could manage. A weighed portion was screened to 2F, placed in a mortar and topped with a golf ball. Ignition-to-landing was timed (later on they included a chronograph for velocity). Longest flight time won bragging rights.
I think they started with something like three grams, then lowered it to two grams when they kept losing golf balls.
Goex 2F was used to provide a baseline. Almost every "golfer" could make BP that was faster than Goex.
For centuries it's been known that the source of carbon makes a huge difference in burn rate of BP, with charcoal generally topping the list. I was surprised to learn that certain types of charcoal are superior even to lampblack (nano-sized particles of nearly pure carbon).
BP manufacturers standardize on a particular variety of charcoal for uniformity. Willow, grapevine, and a few other types produce faster burns. But sourcing large, uniform quantities of these other types of charcoal is something of an impediment to large-scale manufacture.