It could just be the way the shaders pack is designed. That being said, Try editing the shaders settings. If your using Optifine then they should be on the side screen in the shaders tab if not then you'll be able to find that info on the shaders website. If that doesn't fix it try updating your graphics driver. Whenever I used resource packs my FPS would drop, but then I updated my graphics card and the issue stopped. I don't know if it's the same for shaders, but it wouldn't hurt to try. Other than that check your video settings that have to do with distance. I say this because your shaders pack is probably lagging your FPS because of how it loads in the distance. In Optfine under quality settings turn antialiasing to off as well as anisotropic filtering those two have a big impact on fps. If none of those work then I have no clue I can do some more research on your shaders pack, but I doubt the shaders pack itself is the issue.
