In short terms: it isn’t slow 🙂
Compared to an native application it is slightly slower but there should be not that much difference. (This is true at least since version 0.42 of JWildfire at which I have tried to optimize the hell out of it ;-)) For the maximum performance you should use Java 7.
I have created some flames for benchmarking. I used Apophysis7X15 as reference because it is very popular. Below are the results (the used *.flame files are linked inside the table):
Render settings: Resolution: 1920×1080, Filter radius: 0.25, spatial oversample 1
quality 100 quality 500 Apophysis7X15 14.99s 72.61s JWildfire V0.42 18.27s 88.53
quality 100 quality 500 Apophysis7X15 5.99s 28.04s JWildfire V0.42 8.86s 41.632s
quality 100 quality 500 Apophysis7X15 10.69s 51.06s JWildfire V0.42 12.80s 62.51s