Understanding how build time really affects Jekyll workflow

I am building a complex personal website/blog. I am working with Hugo and I am thinking to switch to Jekyll because it has a much simpler syntax. A lot of people complains about Jekyll built time. I am trying to understand how this really affect workflow. Does the slow build only happen when you are building a server ready site? Or does it happen every time you save something while working on the site? (i.e. when you edit markdown or css or HTML etc.)

Any pointers how I can understand constraint of build time in Jekyll will be much appreciated.