Performance impact of rewrite conditions?

Question

My web framework (cakephp) uses the following rewrite conditions and rule:

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php?url=$1 [QSA,L]

I serve up a lot of image files (~10 a second). I'm wondering if it would improve performance to have a rewrite rule that exempted requests for files in my images directory from even trying those rewrite conditions (checking whether the file exists). My traffic is constantly fluctuating, so this would be hard to benchmark, which is why I thought I'd ask here.

Would adding this above the other conditions improve performance to any significant degree?

RewriteCond  %{REQUEST_FILENAME} !^/(images)(/.*)?