My custom css is not cached by wordpress

I am using WordPress with wp-super-cache but my website is still slow. Using online tool i see that the problem comme from this line:

<link rel="stylesheet" id="custom-css-css" type="text/css" href="http://mywebsite.com/?custom-css=1&csblog=1&cscache=6&csrev=52" />

I though wp-super-cache would transform it in a real css file instead of asking database.
The version of the script dont change till 1-2 week.

And what does “cscache=6” stand for?

Kind regards,