JIT warmup
A just-in-time (JIT) compiler turns frequently run code into optimized machine code while the program runs. Until it has profiled and compiled the hot paths, a new process runs below its peak speed, so its first seconds or minutes are slow. The JVM and .NET warm up this way; tiered compilation and ahead-of-time profiles shorten the warmup.
Runtimes
In Quorum#
Each runtime has a start speed and a warmup time. A cold Tomcat (Java) App Server starts at 30% CPU speed and reaches full speed after 30 s; Kestrel (C#) starts at 60% and Node.js at 80% for 15 s, while Go, BEAM and Tokio start at full speed. Every restart, resize, Stack change, and new Autoscaler copy starts cold, so add capacity before a peak, not during it. PHP warms its OPcache instead: from 1× up to 2.5× CPU speed as the cache fills.