One thing I've since hit with this one is DLL in use stuff, so it might be work setting the SkipUnchangedFiles="true"

Agree; however, most time is consumed on force compilation of LibSass project twice in LibSass.Net.csproj, since we are compiling everytime regardless of C/C++ source changes. The real difference at (solution level or even a single dependent project level) build time would be made when we use Rebuild instead of Build target there in MSBuild task which builds libsass. We cannot do that right now as Rebuild whips out the previous build artifacts (x64 rebuild will delete x86 build artifacts, regardless the TargetFile for x64 has different name..).

Another reason not to compile for many platforms at once.

Going forward, build for every architecture at single compile-time is neither feasible to build nor to test (x86, x64 and in future ARM, ARM64 etc. and then OS Linux-glibc, Linux-musl-libc, Mac, Windows etc. in future..) We would need to build one platform at a time (#47). Fixing #47 is next thing on my todo, after this PR is merged.

Yeah, I think probably in Debug it should only compile the current achitecture, and leave the full matrix for the Release. Separate stuff though.

Separate stuff

I have added SkipUnchangedFiles with 16b8f1c.

The problem may still persist as SkipUnchangedFiles checks for modified timestamp to detect the change, which in this case the DLL always gets build and modified so we might not get the desired effect unless we start building for one platform. Hence <MSBuild target="Rebuild" will ultimately fix this problem. :)

Yup, this only helps when your doing a Project Build instead of a Solution build. Thanks for fixing it though!