Serious Sam 3 installed via Steam has serious performance issues: even when the internal fps counter displays values beyond 60, the game is unplayable: it stutters and stalls in a way which makes even seasoned first-person shooters sick.
The solution is simple: install cpupower (if you're using systemd) or cpufrequtils (initscripts) and issue:
cpupower frequency-set --governor performance
cpufreq-set -g performance
The latter command applies only to one core. If you want to set all cores, edit /etc/default/cpufrequtils and restart the service.
Happy fragging! 😉