[jwr]

Compile times for my project are around 2m15s to 2m30s. Since this work involves lots of CPU (some single-threaded, some multi-threaded work) and I/O, the spread mentioned above is what I get with multiple measurements.

While it's true that without the mitigations times were much closer to the 2:15 mark, there were still outliers at 2:24. Which means it's hard to draw any meaningful conclusions.

Not sure where you got the "0.5%" figure from.