I don't think it will affect the cold start period of lambda as you have already installed bundle present. If you were to install at runtime, then the cold start would be longer.

Sometimes packages in a different language can be heavier. The best you can do is use libraries that are lighter and then there is scope for reducing size. However this 9MB size is also fine, no issues with that. We have deployed code much heavier than this and it was not slow.