the key on your keyboard.....thats probably something to do with you.....dont worry, we dont need to know what youve been getting up to in front of your PC and what could have gotten stuck between the keys.....o_o
Try turning the graphics down, and setting it to a lower resolution. What are your system specifications? Also, ESF is much more Power Hungry than Half-Life if youmeant it meets Half-Life's requirements......
For ESF, i would recommend the following for decent performance:-
A GeForce 2 or higher
P3 733mHz minimum if you have a decent graphics card or an Athlon 1 gHz or more
at least 256 MB of DDR RAM, if SD RAM, consider upgrading to 512 if you haven't already
As a last ditch effort, get rid of the background programs that are running in Task Manager.
Also, do you mean the gameplay is choppy when you play online, or when you play a LAN game?