I wonder if it depends on Desktop Environment?
Using Xfce (inside a VM, i7-6600U 2x2.6GHz, 4GB RAM, tmpfs for /tmp) I don't personally see much slowness with Firefox most of the time, however when loading pages with lots of javascript or lots of tabs at the same time there is considerable slowness. But I don't experience the other slowness.
Using Xfce (inside a VM, i7-6600U 2x2.6GHz, 4GB RAM, tmpfs for /tmp) I don't personally see much slowness with Firefox most of the time, however when loading pages with lots of javascript or lots of tabs at the same time there is considerable slowness. But I don't experience the other slowness.