I’ve been playing the Direwolf20 1.7.10 pack of Minecraft from the FTB Launcher, and I’ve been suffering from lag in it for some time. I had chalked it up to my computer and low FPS or to my slow internet connection to the server I play on. However, recently I noticed that it was actually centered around my mouse and keyboard. If I stand still and move my mouse enough for my character to do a 180, my mouse moves a consistent distance each time. In my case it’s about 1 inch. However, if I hold down a button on my keyboard and repeat the same thing with my mouse, I find my character turns wildly different amounts each time. Holding the key down for my push to talk in Teamspeak might mean I need to move my mouse 1 inch to do a 180, and 2 seconds later, I might need to move my mouse 4 inches to perform the same 180. It’s like someone is controlling my mouse sensitivity and randomly changing it from 20-100, but only as long as some key on my keyboard is held down. My friend playing the exact same mod pack, and the exact same server, does not experience this.

After some research online I came across this post talking about the exact issue. It would appear the problem is related to version 2.9.1 of LWJGL (lightweight java gaming library) that is installed alongside Minecraft. Mine was installed by the FTB (feed the beast) launcher, but it’s also installed by other launchers such as MultiMC or even Mojang’s official launcher. This issue also affects multiple versions of Minecraft as well. So basically a whole lot of people may be impacted by this problem. According to the post on their site, this bug is fixed in version 2.9.3 of LWJGL. Eventually all launchers will probably go to 2.9.3, but until then a manual upgrade is required.

The launcher I’m using as stated above, is the FTB launcher. It places these LWJGL files in 3 locations
\FTB Launcher\libraries\org\lwjgl\lwjgl\lwjgl\2.9.1\lwjgl-2.9.1.jar
\FTB Launcher\libraries\org\lwjgl\lwjgl\lwjgl_util\2.9.1\lwjgl_util-2.9.1.jar
\FTB Launcher\libraries\org\lwjgl\lwjgl\lwjgl-platform\2.9.1\lwjgl-platform-2.9.1-natives-windows.jar

Those are the 3 files that need upgraded to fix the lag issue. I was able to download 2 of the 3 files from the LWJGL Sourceforge page. You need the following 2 files from the zip.
lwjgl.jar
lwjgl_util.jar

The third file I found at the following website
lwjgl-platform-2.9.3-natives-windows.jar

The 3 files are then renamed to match the original file names and used to overwrite those original 3 files.
lwjgl.jar is renamed to lwjgl-2.9.1.jar
lwjgl_util.jar is renamed to lwjgl_util-2.9.1.jar
lwjgl-platform-2.9.3-natives-windows.jar is renamed to lwjgl-platform-2.9.1-natives-windows.jar

After the 3 original 2.9.1 files are replaced with the 2.9.3 versions I fired up Minecraft and my input lag was completely gone. If you want to get the original files back all you have to do is delete the new files and the FTB launcher will automatically download them again.

Take careful note that we are downloading the 2.9.3 versions and then renaming them to be exactly the same name as the original 2.9.1 versions. Then we overwrite the original 2.9.1 versions. This is to trick the FTB Launcher into thinking it has the correct 2.9.1 files it is expecting. If you have a different launcher you will have to figure out where the LWJGL files are located.