That still isn't a very specific answer. "Significant resources" as reported by unnamed performance profilers is hardly damning evidence. I'm not saying s2Member isn't having any affect on your site's speed, I'm just saying that it probably isn't the culprit you are being told it is.
If you truly want to see what affect s2Member is having on your site speed, you'll need to test it against gtMeterix or PageSpeed with s2Member activated and without it activated. Make sure you actually have some pages that are restricted, but don't test it against restricted pages because the service won't be able to access them. You just want to see if s2Member is significantly slowing down your site when checking restrictions: which would be the MOST intensive active s2Member use of resources.