While looking at dotnet/fsharp#14273 I compared a non-source-build to a source-build and the big chunk that stuck out was a restore time of about 2.5 minutes for the source-build vs about 30 seconds for the not-in-source-build-but-source-build-leg build. SDK also spends most of its source-build time in restore but seems to be hooking in and doing a lot more work there than the average repo. I think NuGet restore might be running into issues with the size of our packages blob feed but I think this warrants looking into further.