Now, the average page load time for Wikipedia is 1.9 seconds.

Saving .4 seconds over the day means each user will save .000000008 seconds.

Noticeable? No.

Another way to look at it is at their current rate, by saving .4 seconds per day, they get one additional page load every 5 days for the same exact amount of processing time it took them to do it otherwise.

Now put that into a script being written and run for joe schmoe developer who's site gets 12,000 hits per year. It'll take that site 41,666 years to see that same .4 second save in time.

$world = 'world';
echo('Hello ',$world,' !');

Since using commas doesn't require PHP to do any concatenation and just passes 3 parameters to echo.

$world = 'world';
echo("Hello $world !");

and:

$world = 'world';
echo('Hello '.$world.' !');

I strongly suspect that there would be very little difference between them but you just can't be sure until you have actually tried it.