I made an alt recently and it went from lv 1-50 in hours wearing the Endwalker pre-order earring. Granted part of it was knowing about things like hunting log, enemy/quest locations, and skipping a number of CS I had seen previously (I watched a few because I started this alt in a different city). I didn't hit a noticeable slowdown until I cleared Praetorium which capped off the initial ARR story and the MSQ stopped handing out large amounts of EXP. This likely is just due to the game's lvl cap not changing until HW so I'll see if it picks up again once I actually hit HW with that alt as I've been doing other stuff on my main to prep for Endwalker.