Everything is taking the same amount of time (10 seconds) to cook in the Kiln as in a furnace. The JEI tooltip states that it should take 5 seconds.

Just an update, I tried fixing this by adjusting the cooking time in a data pack, JEI now says 2 seconds and it's still taking 10 seconds. My guess is, the Kiln is currently hardcoded to use furnace cooking times for some reason.

Nope, this is an issue caused by performant