What exactly causes shader cache stuttering?

[Fiora]

Writing to the disk is always async unless you flush afterwards; the OS handles buffers in between and will often write it seconds later than you actually requested the write. it's reading that's inherently synchronous by default.

Sharing shader sources might be possible, but they can change significantly between Dolphin versions, and Dolphin has many workarounds in its shader generation that depend on the precise features supported by a given graphics driver, or even based on the current graphics settings (e.g. "per pixel lighting" changes shader generation, IIRC).