The NVM init slowness issue isn't specific to WSL - see https://github.com/creationix/nvm/issues/1277. I also had to sandbox the NVM init lines (on macOS) as timing my .bashrc revealed it as the worst offender.

On the topic of speeding up terminal startup, I can recommend https://github.com/benvan/sandboxd for managing various shell initialization scripts to only be run or sourced at the point where they are needed.