Very slow site front and back

I have a lot of garbage js and css and header problems. It has all been working okay and no complaints from customers. But lately the backend and frontend is super slow. I have been deactivating plugins but still find it very slow. Pagespeed shows disastrous results.

What can i do? Except starting over clean and only use the plugins and content that I now know I need?

I do not feel hummingbird or smush is helping. In face I am thinking that hosting might be one difference. Can you help me check where the bottlenecks are? What I can do about it?