Information
Ever since the game first came out there have been reports of people getting horrid stuttering. At first, everyone thought it was GPU related. This was, in most cases but not all, wrong. The real issue stemmed from the game using 100% of the CPU and this seemed to only occur on dual core CPUs.

The Fix [For V1.1 and BELOW]
The first thing you want to do is navigate to your Steam folder. From there just follow through to steamapps>common>thief>Binaries

Once you are in the Binaries folder you will see two folders inside; win64 and win32. Simply change win64 to any other name, a good one is backup. Then change the win32 folder's name to win64 and you're done.

The Fix [For V1.2 and ABOVE]
Simply open the launcher then click on "options" then checkmark the box at the bottom of the newly created window that says "32-bit".

Additional Information
Despite the fact that the developers stated that this fixing method will do nothing for higher-end machines, I have recieved many reports contradicting this. Even on a higher end machine this fix could give you up to a 15 FPS boost.

Information:
The optimizations in this sections aren't needed by very many PCs. However, if your PC is under the minimum required specifications or you just have low FPS, then this is for you.

The Fixes:
Navigate to your Steam folder then go to steamapps>common>Thief>ThiefGame>Config

Now, locate the file named ThiefEngine.ini and open it in any text editor. Use a search/find function to locate the words "SystemSettings" just as printed but without quotes. ( the search/find function can be used by pressing CTRL+F in most text editors on Windows PCs )

In this file you can edit your graphical settings more advanced than just using the GUI in the launcher or in-game. I will only go over which options to turn off if you wish to have better FPS.

Change all or some of the following to say False

bUseHighQualityDoF=False - Increases the quality of depth of field effects

UseHighQualityBloom=False - Increases the quality of Bloom

Distortion=False - Adds effects that simulate what happens when light bends ( Just think what happens to your reflection in a spoon )

FilteredDistortion=False - Just filters distortion effects, using up slightly more processing power

DropParticleDistortion=False - More distortion effect things, not exactly sure what this does

FogVolumes=False - Makes fog look better