Run, do not walk, run, to add this to your Apache config file:

php_flag zlib.output_compression On
php_value zlib.output_compression_level 5

I just tried this and achieved 10x and 15x speed inprovement on
some mature php pages.  Pages I have been seating over to make 5% gains on.  I use microtime() on critical pages to help me track page speed and that confirms the speed improvement.  The php page takes a timestamp at the beginning and end, then logs the page duration.  So any IP transmission effects are not included.  There is
a clear subjective difference to the user.

The test system was PHP 4.3.6, Apache 2.0.49 over Linux 2.4.
As always YMMV.