The only good and effective similar plugin i found was the discontinued MN Combine.
Now i will not use any cause my site is fast enough, i don`t use any caching plugin either.
The site has too many requests though and your plugin make them much less, so i was really surprised to see worse times in pingdom.
I don`t know what could have cause it.
well, you might in fact have fallen victim to a bug in 2.4.0 which in certain situations (PHP versions without mbstring support but with iconv) cause significant delays (due to iconv being _very_ slow). this was fixed in 2.4.1 (release earlier this week), so you might want to give this one more shot @satsoklgr ??
ok, i tried it.
In page source everything looks beautiful, css and js files are combined and the site loads fast and correct.
But one time the load time is 5 seconds and after a few seconds the next test shows only 2 seconds.
These pingdom servers(as every server) are up and down and not to trust.
Anyway, the plugin seems to work and to be fair i have to change my first vote!