A specific Safari WebKit page on iOS - how to see memory usage graph in Instruments?
I would probably consider Instruments job done once you know mobile safari is allocating 640 MB of ram and move on to Safari web inspector. Setting it up is quite easy and since you have XCode and macOS safari, you have first party tools to debug your process while it runs in real time. 


https://developer.apple.com/library/content/documentation/AppleApplications/Conceptual/Safari_Developer_Guide/GettingStarted/GettingStarted.html


The activity viewer will right away focus your efforts based on whether you have excessive warnings and errors or simply are loading a lot of assets.