There is a browser, Kiwi Browser, which is based on Chrome and has built in ad and pop-up blockers and allows you to use a lot of desktop Chrome extensions. Now for a disclaimer - I have not tried it, so can't tell you how good, bad or indifferent it is, but there is a review of it at a site I trust here
https://www.techsupportalert.com/content/kiwi-browser.htm
The other thing I would suggest is going through the individual apps on the phone and deleting any cache or memory they are using which seems excessive. No idea how you do that on 6.0.1 as I've never used that version of Android, but the details will be available somewhere online.