Fps drop with shader

Depends on what shader you are using, which settings you have and what amount of ram you have.
Try using optifine if you arent already
allocate more ram to mc. to do this in the launcher go to launch options enable JVM arguments and replace the "-Xmx2G" with an appropriate number depending on how many gb of ram you have (substracting 2Gb for system and other applications) so if you have 32gb like me you would replace it with "-Xmx30G"
also check what settings the shader is at. to do this go to the shader menu and click shader options at the bottom right. these settings differ greatly from shader to shader so i cant give any specifics that would help but usually lowering shadow resolution helps a bounch