Nintendo's CDN for Wii U and 3DS digital games is rather poorly secured, due to a huge oversight when designing the software protection for these consoles. As such, with tickets proving you bought the game (but which you can get through less legit ways), there are downloader utilities like NUSGrabber, FunKiiU, UWizard, or jnus among others, and decryption tools like UWizard and CDecrypt, to obtain a functional copy. The process of how to exactly acquire those files is detailed elsewhere on this wiki.
When your game is loaded, check for its ID on the title bar in front of "SaveDir:" and note it down. The directory where transferable shader caches are stored is .\Cemu\shaderCache\transferable. They're generated the first time you play through a game and at that time cause noticeable stuttering. You can download a complete cache from places like here and rename them to match your game version's ID to spare yourself most of that unpleasant experience. Just keep in mind that shader caches from versions older than 1.8.0 are incompatible with later versions of Cemu.
Some games' transferable caches may cause slowdowns on some graphics cards like the GeForce GTX 970. If this is the case, you will have to generate your own shaders and delete the ones you downloaded. In the case of the GeForce, also consider deleting the .bin and .toc file in your GLCache at %APPDATA%\NVIDIA\GLCache\(hexstring)\(hexstring).
Since 1.13.0, the graphic packs system and interface was overhauled for better use and many bugs when using multiples graphic packs were fixed.For 1.15.1 and now on, users can download the community's graphics packs linked below directly from Cemu user interface and just check the graphics packs they want to enable. That update completly removed the need to manage files for the user. 2b1af7f3a8