Linus wasn't very clear, but the issue has more to do with the cost of context switches between the kernel and a userland process to support a userland filesyst

Linus wasn't very clear, but the issue has more to do with the cost of context switches between the kernel and a userland process to support a userland filesystem. These switches are relatively expensive; a good example is the relative throughput of a LUKS/DM-CRYPT encrypted volume compared to a Truecrypt volume with equivalent cipher selections. LUKS' throughput is much higher, simply because it does not add at least two more kernel<->userspace transitions on every read and write.