Go modules builds are slow: please use a proxy (e.g. athens, goproxy)

Every time go test runs in module mode in Travis, go test will download every package from the internet. This takes time. Ideally an Athens server would be run by Travis co-located with the builds and GOPROXY would be set to point to that.

Alternatively Travis Go users could go mod vendor every time one of their dependencies change, and explicitly set GOFLAGS=-mod=vendor (see go issue) but this seems like an annoyance that’s also likely to cause issues (when people forget to do that).

The fact is that the default behavior in the 99% case (where everything goes right with modules instead of failing due to a mismatched hash - which would also be resolved by Athens) is really slow.