ベストプラクティス

No, the interpolation of double quotes is NOT significant.

This is because single quotes must also read the whole string, char by char, looking for:
\'
\\

The double-quote version merely has a larger number of 'case' statements to apply for $var and embedded hex values.

The expensive part, the iteration, is the same for both.

Try this site to check the speed of single quotes versus double quotes:

http://www.phpbench.com

I've found that it has a lot of best practices to help you out as well.
One major one for me was counting the length of a loop before hand instead of doing it inline:

$len = count($arr);

for ($i = 0; $i < $len; $i++)
{
// Code
}

Is faster than
for ($i = 0; $i < count($arr); $i++)
{
// Code
}

Micro-optimization like "single quotes are faster" is unnecessary!

Interpolation makes script slower, it matters! Single quotes are faster!

Double quotes interpolate, single quotes don't. Isn't this important?