There's no way that your computer wouldn't run ET smooth. try changing the max fps to 125. i used to get lag whenever i uncapped them . and also you should follow the advice of running ET with just 1 core which is enough. Make sure that you have no anti-viruses /firewall interrupting.
If it didn't work then let's believe that it's PB related. try the following cvars:
pb_security 0
pb_sleep 500
pb_system 0
Some times you get fps drops caused by interruptions on your sound card, remember that ET is really old and there's a chance that the sound filters aren't the most optimal for new age sound cards.
If it's the case then the following cvars should make a difference.
s_mixPreStep "0.05"
s_mixahead "0.2"
s_doppler "1"
s_separation "0.5"
If it didn't help either then let me ask you, did you copy paste that ET installation? If so there's an issue, whenever a profile is created an ET cfg is also created on each folder which includes everything related to your gaming interface /connection/ hardware, many times FPS drops are caused due to a command called r_lastvalidrender which sets the GPU that you were using when you created the profile. So delete all the profiles in your ET installation and try again.