Yeah, I agree with Ashwanth.
You'd need to share more details, like the logs during start-up, version number, nature of your deployment, list of the plugins etc and details of your analysis - and clarify whether you are talking about the server restart time or an agent. There's not enough information here for anyone to help you. Maybe look at the GitHub issue template to get any idea of what you might need to share to get help:
https://github.com/gocd/gocd/issues/new While plugins may not be the root cause, are some of these plugins custom internal ones? 18 plugins is generally quite a lot in GoCD context, which makes me think some are possibly proprietary.