Yeah, try pressing Alt+[PrintScreen, F]
to invoke the OOM killer. It kills the memory-hoggingest process, usually the web browser.
Fedora documentation says this sysrq functionality may be disabled by default. You can enable it once by typing at a terminal: echo 1 | sudo tee /proc/sys/kernel/sysrq
or permanently with echo 'kernel.sysrq = 1' | sudo tee /etc/sysctl.d/90-sysrq.conf
If it turns out that memory overconsumption is the problem, you can sometimes fix this lag by disabling swap. 16GB is easily enough RAM to do all normal desktop things.
I bypass this using Firefox’s “reader view”. The button to toggle it is at the right edge of the address bar. It removes many annoyances from badly designed news sites.