You can take a look at Sleek Render Sleek Render - Lightning Fast Mobile Post Processing Effects
I have quite a few ideas explained and a few references there in the topic, so you’re not even forced to buy it if you can implement everything yourself.
I don’t think Unity will optimize their buit-in PP stack any time soon because they already have some conditional expressions like #if MOBILE in their source code, which does have some minor performance improvements, but not much really. I also noticed that in some cases they put #if MOBILE OR CONSOLE there so they treat mobiles and consoles as equals in some cases, which is not always convenient.