As the title says,
I currently have a website with https on sensitive pages and cached http on the rest for speed.
I know HTTP2 is faster than HTTPS. What I don't know is whether HTTP2 is faster than regular un-encrypted HTTP?
Would I see performance improvements if I encrypted everything with SSL and enabled HTTP2, compared to using HTTP without encryption but with caching?
It completely depends on your site.
However, saying that, there is practically zero noticeable speed penalty for using HTTPS nowadays - unless you are on 20 year old hardware or serving huge content (e.g. you are a streaming site like Netflixs or YouTube). In fact even YouTube switched to HTTPS for practically all their users: https://youtube-eng.blogspot.ie/2016/08/youtubes-road-to-https.html?m=1
There is a small initial connection delay (typically 0.1 of a second) but after that there is practically no delay, and if on HTTP/2 then the gains that m will give to most sites will more than make up for tiny, unnoticeable, delay that HTTPS might add.
In fact, if you have some of your site on HTTPS already, then either you are using HTTPS resources on some of your site (e.g. common CSS that both HTTP and HTTPS pages use) and already experiencing this delay, as you need to connect over HTTPS to get those even when on HTTP, or you are making them available over both and making your HTTPS users download them again when they switch.
You can test the differences with a couple of sample sites on my blog here to give you some indication of differences: https://www.tunetheweb.com/blog/http-versus-https-versus-http2/ - which is a response to the https://www.httpvshttps.com website that I feel doesn't explain this as well as it should.