That didn’t help either. But I now did notice a crucial difference between the previous fast builds and the slow ones now:
Bot cases it finds versioned dependencies very fast, e.g.
go: finding github.com/AudriusButkevicius/go-nat-pmp v0.0.0-20160522074932-452c97607362
In the fast cases it was done then. In the slow cases it does pull the latest version too, and that takes a long time:
go: finding github.com/AudriusButkevicius/go-nat-pmp latest
The only thing in the release notes that might be related is the part about version validation (in https://golang.org/doc/go1.13#modules), but I fail to understand how that would result in go pulling the latest version of the module.