Well since you didn't say what the FUSE filesystem was or what it was doing it's tough to say anything too definitive but if you think about it you're still making syscalls into the kernel (e.g. through your libc's
write call) which in turn will have to transition BACK to userspace to talk to the FUSE filesystem. So:
Is performance of FUSE based applications effected [sic] with Kaiser patch in Redhat [sic]?
Yes, FUSE performance will be affected by the Kaiser patch on Red Hat because you're likely to make at least user-kernel-user-kernel-user transitions when your user program is operating on a FUSE file (see the Direct-FUSE: Removing the Middleman for High-Performance
FUSE File System Support, To FUSE or Not to FUSE: Performance of
User-Space File Systems papers and the "Comparison of kernel and user space file systems" thesis (chapter 4.1.1)). You're correct that FUSE filesystems run in userspace but typically as a different process to the one performing the operations (yes, a program might be able to talk to a FUSE filesystem by using it as a library but this is not the typical case because the filesystem code will have to have been written with that style of operation in mind and without
LD_PRELOAD tricks, programs will have to be changed on a case-by-case basis to use it in that fashion).
Will it have a drastic performance impact with the Kaiser patch?
Maybe? :-) Impossible to say for sure - it will depend on your workload, your hardware and your configuration.