As a hiring manager for nearly 4 years straight, dealing with way way more than 100 applicants for some positions, I know it takes minutes at most.

All hiring systems have ways to send batch emails to rejected candidates.

If you don’t have a hiring system for some reason, it’s still just hitting reply/ctrl-v/send to each applicant you move out of the “possible candidate” inbox.

Giving a reason “why” tends to hit people badly if they didn’t specifically ask, so a stock response is not only easy to give, but the best response. Whether and how to respond in more detail to people asking for “why”, is a less easy decision but good if you are able to.