Initially, enabling GZIP compression with this plugin made my site take 1 second *longer* to load - both on first byte and "fully loaded."
Is it supposed to get better with time?? (As it is, I disabled the plugin as soon as I got back test results from https://gtmetrix.com/ and http://www.webpagetest.org/
Both tests yielded similar results.
I'm a bit frustrated because GZIP compression is enabled on the server, yet the tests say my site is not GZIP enabled. Now, even enabling with this plugin got similar results, except that load time takes 1 second longer.
This evening, GMetrix reported: Fully Loaded Time 6.9s Total Page Size 742KB with plugin enabled. Disabling plugin reduced time to 5.8 s.
webpagetest.org reported 6.228 s with plugin and 5.167s without plugin. So in each case, the savings was over 1 second for deactivating the plugin.
PS I notice that some of the 5-star ratings should be 1-star because they indicate that the plugin is not working.
I found out that the "deactivation" of the plugin didn't actually "take."
When I re-tested my site (over 200,000 unique visitors a month with Wordpress) this morning, I was surprised to see that the testing sites showed "gzip compression enabled." And, sure enough, WP showed it still enabled.
Turns out that was a good thing. This morning, average full-page load time is 3.5 seconds. But I've had that before in slow times, even wo gzip compression, so I'll reserve judgment till I can test it again at a busier time.
Take-away from my experience: It takes a while for the server to implement efficient gzip compression. So give it a bit of time.