The entire site runs in https mode for usability reasons. It's very common for Pressero users to include links to images and external scripts on their sites. If the site switches between http and https, the links to those external resources would also need to change. We can't control whether those external resources are available in both modes... and dynamically rewriting url's that our customers have added to their sites can have negative implications too. So, after some initial experimenting, we found that this approach works best for most Pressero customers.
You should be able to achieve a fully compatible ssl site. If your browser is reporting problems, try investigating the html source to see what might be going on. The most common issue is hardcoding urls (in the admin) using http rather than https. And if you can't figure it out, please do submit the issue to support.
SSL does have a slight performance impact... but it really is slight. 5-10 years ago this was a bigger concern. But CPU's have exponentially increased in power. The main concern is at the server level. As I type this response, I just checked the CPU load on one of the Pressero servers and see it hovering around 10%, which is typical.