What is shader compilation jank?

A shader is a piece of code that runs on a GPU (graphics processing unit). When
the Skia graphics backend that Flutter uses for rendering sees a new sequence
of draw commands for the first time, it sometimes generates and compiles a
custom GPU shader for that sequence of commands. This allows that sequence and
potentially similar sequences to render as fast as possible.

Unfortunately, Skia’s shader generation and compilation happen in sequence with
the frame workload. The compilation could cost up to a few hundred milliseconds
whereas a smooth frame needs to be drawn within 16 milliseconds for a 60 fps
(frame-per-second) display. Therefore, a compilation could cause tens of frames
to be missed, and drop the fps from 60 to 6. This is compilation jank. After
the compilation is complete, the animation should be smooth.