This is a mod aimed at improving performance of all crafting-related functions. It fixes the bug introduced in Minecraft 1.12 where shift-click crafting a stack of items could cause momentary freezes. However, it does slightly more than just fixing that issue.
FastWorkbench causes all crafting operations to cache the last recipe used. On top of this, it reduces calls to that cached recipe, by only detecting changes when the stored item in the crafting matrix actually changes (vanilla retries every time anything changes, such as stack size). This results in the number of matching operations being reduced to one, instead of anywhere between one and nearly 600.
Back in 1.12, FastWorkbench fully removed the recipe book, which meant recipes were no longer forcibly synced on login so that the client could see them in the book. This resulted in a huge boost in login times. In 1.13 and higher, the recipes are synced anyway as part of the new recipe system. In these versions, FastWorkbench visually removes the recipe book button by default, but it can be configured back on.
Compatibility and Mod Support
This mod is applied via Mixin to the classes PlayerContainer (for the 2x2 grid) and WorkbenchContainer (for the 3x3 grid). Any mods utilizing WorkbenchContainer will receive the full benefits of FastWorkbench.
Any mods not utilizing WorkbenchContainer should probably be performing these optimizations on their own already - they are fairly trivial to add.