I decide they are the best candidate. A recruiter talks with them to negotiate compensation and they accept the offer. This takes a week at best, but can take weeks if they are choosing between multiple offers. Then they choose a start date. They've got a couple weeks at the old job, plus probably some time in between roles before they start. So 2-6 weeks waiting here. Then they join and go through the company-wide onboarding and training processes and set up their equipment. Another week.

The first time I actually get to have them do any work is 4-10 weeks from the date I chose to offer them a job. It now takes me some time to realize they are hopeless and misrepresented themself on their resume. Three weeks would be an extraordinary outcome here, but it more likely that this takes 8+ weeks. Even if the actual process of firing them is instant once I've decided that it was a bad hire, I'm still out 3-5 months from the date I chose to hire them. Any other strong candidates I had in the pipeline now have other jobs and I am starting from scratch.

That is incredibly expensive.