forceHTTPS and session()

I have login page and user administration. It calls secure page, while other page is not encrypted to lighten processor load.

I don't have any answer to your specific problem, but there are no performance impact with HTTPS vs HTTP that can't be solved with some itsy bitsy upgrade (or non at all). We are talking about max 5% CPU increase. If you configure your server correctly.

So go full HTTPS and come back if it still loose the session. Don't know what kind of site you have your users submit any kind of data with HTTP those will be unsecured.

EDIT: With encrypted connection you can also enable HTTP 2, making your website appear faster to your users. As it allow parallel downloads.