Why system calls via interrupts are slow and thus we have SYSENTER/SYSEXIT instructions?
There is the general rule about interrupts being slow, and the particular case of x86 CPUs.


x86 have had several ways to enter kernel mode : Call gates, interrupts. They are complex, microcoded instructions, requiring many register save operation and protection checks. SYSENTER/SYSEXIT are just optimised methods : For example, since the 80286, 4 protection levels were available, but rarely used (except, IIRC, by OS/2 ?) : SYSENTER/SYSEXIT support only ring 0 (user) to ring 3 (kernel)
On many instruction set architectures, software interrupts are used to enter kernel mode from user code. These instructions can be optimised, but, in the simplest implementation, they are handled like external hardware interrupts or MMU faults. They are slow because the pipeline must be flushed, prefetched instructions must be discarded, before jumping to the interrupt vector.