What I did for my iPad2 was to disable js on safari, which makes a big part of the web fast and usable again, in combination with reader mode. I had Brave istalled for using websites more interactively, and for sites like youtube. It has adblocking without any hardware requirements. If Brave still supports ios 9, you could try that. It will still lag, and certain apps were simply unusable for me (protonmail), but I increased its useful lifespan this way.