Our cold-start times have a way to go, they're around 1 minute on the first invocation of some set of binaries and less than 20 seconds on subsequent invocations.
We've done a lot more work on optimizing the subsequent invocations time, including even running a forked version of docker (it reduces docker save time by 100x or more)
This is a primary scenario for us though and there's a lot of room for further optimization. We'll be posting figures and progress regularly post-launch.
EDIT:
I missed some of your questions, we need to upload your project's binaries only (not package manager binaries). So languages with larger binary sizes will have longer cold start times. Overall this comes down to the factoring of the project though.
We should be able to support Go since the binary is static linked, but it has less portability than other managed languages so we've targetted those first as lower hanging fruit.