[n3t]

You might want to use "--unbuffered" flag in sed execution.

[sheepz]

that option appears to be gnu-specific. -l should make it line-buffered

[masklinn]

Note that -l is also an extension, just a BSD one.

The only options in POSIX sed are -e, -f and -n (and -e is usually cargo-culted by people who want -E. -E being a BSD extension enabling extended regular expressions — -r in GNU sed)