Depends on the workload.

Normally you would go read() -> write() so:

1. Disk -> page cache (DMA)

2. Kernel -> user copy (read)

3. User -> kernel copy (write)

4. Kernel -> NIC (DMA)

sendfile():

1. Disk -> page cache (DMA)

No user space copies, kernel wires those pages straight to the socket

2. Kernel -> NIC (DMA)

So basically, it eliminates 1-2 memory copies along with the associated cache pollution and memory bandwidth overhead. If you are running high QPS web services where syscall and copy overheads dominate, for example CDNs/static file serving the gains can be really big. Based on my observations this can mean double digit reductions in CPU usage and up to ~2x higher throughput.