StackOverflow でも、Windows上のgccの遅さは指摘されている：

https://stackoverflow.com/questions/8194954/is-there-any-performance-issue-between-cygwins-gcc-over-msvc-compiler-on-window
>GCC on Windows (especially Cygwin distribution) is damn slow in compile speed,
>but I guess this is expected. GCC is cross-platform, has about 5 middle phases
>(transforming from one tree to another), has pluggable architecture, and many
>other things that could sacrifice compile speed for flexibility.

「Windows 上での GCC(特に cygwin 上で）は、コンパイル速度が物凄く遅い。」

damn slow = 物凄く遅い。