Under “What to scan:” click on “plugins” and then check the box for “mailchimp-for-wp” in the popup for “Only Scan These Folders:”.
You could try increasing the memory limit to 256M but normally 128M should be enough. Maybe it’s having trouble with those files for some other reason. It is a pretty unusual that it would take 30 minutes just to scan the plugins directory. Your server may just be really under-powered regardless of your php.ini settings.
(I do my best to consider the efficiency of the scan process, however it is always going to be taxing on your server to read and parse every file looking for malicious patterns in the code. As a comparison to your server, it take less than 10 minutes to scan the plugins directory on one of my sites and I have over 60 plugins, with almost 4000 files scanned.)