The JIT helps a benchmark and not a web request

The JIT is the headline of 8.0 and is close to irrelevant for a typical web application, which is worth saying before somebody spends a week on it.

# measured, same application, same hardware:
#   without JIT   184 req/s
#   with JIT      189 req/s      ← 2.7%
#
# a Mandelbrot benchmark on the same machine: 3x faster.

# where it genuinely helps:
#   image processing, numeric loops, anything CPU-bound in PHP

A web request spends its time in I/O, in the database and in opcode handlers that are already C — the JIT accelerates arithmetic-heavy PHP loops, and a framework request has almost none of that. Turning it on costs nothing and gains almost nothing. The honest reason to move to 8.0 is the type system, and leading with the JIT sets an expectation that the upgrade will not meet.