The GPU (Graphics Processing Unit) is the part of the Hardware that processes everything in terms of graphics on the screen, as well as the RAM (Random Access Memory) which loads everything on your computer running on that current moment. Also the CPU (Central Processing Unit) which controls the core(s) .
The computer has to process every block at the same time as well as entities (Dropped Items, Mobs and Players)
The computer does a lot.
You installed shaders which EVERY BLOCK interacts with, each block changes the lighting.