I did that and sped up by about 50%. Also: It appears that less partials = more speed. I moved all partials like post-head, meta lines and so on into their respective parent templates (home.html, list.html, post.html etc.)
One thing I am not sure about but it felt as if things sped up: removing Go template comments ({{/* commen */}}) containing Go template code ({{/* {{ some Go tag }} */}}) made things faster. My guess is that the inner code is executed and then ignored… or it’s just my imagination.
I am now at around 15 seconds per rebuild.
PS: and I am aware that this post is way off-topic by now.