From Martyn Plummer, on the JAGS news blog. Key graph below, showing a few outlying cases in which JAGS is substantially slower than OpenBUGS, but generally, JAGS performs quite favorably.
Key point from Martyn:
Incidentally, these figures are for JAGS with the glm module loaded. The glm module is not loaded by default. If you are not using it for generalized linear mixed models then you might be missing out.