From the source
Vercel announced that its Fluid compute platform achieves zero cold starts for 99.37% of requests, with fewer than one in a hundred requests experiencing a cold start.
The approach combines scale-to-one (keeping at least one instance warm), Fluid compute's concurrent request handling, predictive scaling, bytecode caching, and rolling releases.
Scale-to-one applies automatically to production deployments on Pro and Enterprise plans, and to the most recent branch deployment for Enterprise plans, with no additional cost beyond usage.
