JS VMs try to be really good and smart about startup time like when you visit a page for the first time. The main trick is to interpret the short running code.

JS VMs try to be really good and smart about startup time like when you visit a page for the first time. The main trick is to interpret the short running code. That said, some JITing makes page load faster. But only if the JIT only kicks in for functions that run more than some amount of time and the JIT is very cheap to run.