Will installing Cyanogenmod speed up my Galaxy S?
In theory, your phone should perform considerably faster with CyanogenMod 11, because KitKat was designed with entry/mid-level phones in mind; requiring only 512 MB of RAM to run. (See this page about Project Svelte for more information.)

CyanogenMod also adds a whole slew of features like Heads Up Notifications and Privacy Guard on top of the AOSP code, so your phone will not only be fast but also be very useful.

Keep in mind, however, that CyanogenMod (along with others such as PA and OmniROM) are third-party ROMs, and therefore lack the various "enhancements" that the manufacturer has made in the stock ROM; so don't be surprised if you see that the camera doesn't perform as well, the battery drains faster than usual, etc.