Here's a simple change in src/buffer.c that reduces the time to six
seconds or so, but only for newer versions of FreeBSD.

It takes advantage of the MAP_EXCL flag for mmap(2), which has been
recently added[1] and is also available in 10-STABLE and 10.1-RELEASE.

In percentage of user CPU time, the hotuser script[2] from the dtrace
toolkit shows a change from

[...]
emacs-25.0.50.1`decode_coding                             537   0.1%
emacs-25.0.50.1`produce_chars                            2109   0.4%
emacs-25.0.50.1`decode_coding_charset                    2544   0.5%
libc.so.7`memcpy                                       516884  98.9%

to

[...]
libc.so.7`memcpy                                          220   4.1%
bootstrap-emacs`decode_coding                             488   9.0%
bootstrap-emacs`produce_chars                            2100  38.8%
bootstrap-emacs`decode_coding_charset                    2501  46.2%

(the second column counts sample points, of which there are 1001 per
second for each CPU core)

The numbers are for the system compiler (clang 3.4.1) with default
optimizations, though they are even a bit better for gcc 4.9.

However, if the file in question is compressed
revert-buffer-with-coding-system still takes 4 minutes (the user time
being dominated to 98% by memmove).

[1] https://svnweb.freebsd.org/base?view=revision&revision=267630
[2] https://svnweb.freebsd.org/base/stable/10/cddl/contrib/dtracetoolkit/hotuser?revision=256281&view=co