In my experience, extensions that mess with scrolling can cause significant lag.
Even though the CPU utilization shows that you are CPU limited in this case, you still probably would benefit from an SSD.
Browsers have their own page cache with a completely different purpose (minimize network activity) from the OS pagefile and disk cache, and is not a function of being low on memory. When you hit the disk with a bunch of simultaneous browser cache reads, the head is forced to seek back and forth repeatedly, and throughput tanks. OS disk cache can help with this, but only when the pages/content in question are "hot" in the cache.
Another commonly-overlooking factor causing slow browsing is that by opening multiple tabs from the same server (are they from the same server), you're opening a large number of simultaneous connections and triggering throttling. Throttling when many simultaneous connections from one client address are seen is common both to protect against denial of service and to share bandwidth more fairly.