To my understanding, exokernels expect hardware drivers, filesystems, and other abstractions to be linked directly into user-space programs, so there is no IPC or context switching in those layers. Application optimizations can therefore drill to any abstraction depth to skip and/or cache more levels of processing and decision making than normal abstraction layers. The kernel security is only permission to hit the hardware, or some portion thereof.
However, the organization is fairly similar to microkernels. One could consider exokernels to be a particular (and peculiar) optimization of microkernel architecture.