Bah I can never seem to reproduce these results 
So checking out channel locally I get:
| what | time |
|---|---|
| release full build | 4.91 |
touch flavors/mod.rs release build |
0.83 |
| release + 16 CGU + ThinLTO | 4.33 |
touch flavors/mod.rs thinlto build |
0.62 |
Just to confirm, you were measuring the time reported by Cargo, right?