There's a trade-off here.
If you decide to move everything to https then people will have a longer waiting time initially in order for the security certificate to be recognized by the client browser.
For now, make one URL the secure version of that URL. Make it start with
https:// and run that URL through webpagetest.org and you'll notice either a purple or pink bar attached to other color bars in the initial waterfall chart next to that URL. That pink/purple bar represents the time for a secure connection to be established. This extra time does not exist in http.
At least with
https:// the odds of a hacker breaking into your site will not be as good since data is exchanged over a secure channel between your browser and server.
Also, Load time is a factor in SEO because google hates sites that take forever to load, and if I'm not mistaken, a TTFB (time to first byte of data) that measures over 1/5th of a second is bad news. This can also be measured on any of your pages via webpagetest.org.
So my advice is, if your site is informational only and doesn't have sections only for special users, then go with http.
If you're storing user information or doing anything that the world should not access (for example, going past a login screen to a special page with the correct username and password), then you're better off using https for those specific pages.