I have a certain place in my world where I can
literally walk across the floor in one level of my mine
and the frame rate will drop to next to nothing (under
20 fps), where the framerate is normally ~250-300+ fps.

I noticed this started happening around the same time as
the 1.5.1 release and/or Java 17 came out.

Something I noticed while trying to get enough info to
post a "reasonable" bug report...

I had Windows Task Manager, Performance tab (showing
CPU utilization graphs) running on my 2nd monitor while
playing MC on primary monitor...

While framerate running "normal" in the 250+ range, I
see 4 cores with consistant utilization > 20%, 3 of
them in the 15-30% range and one core in the 80%+ range.

When I walk across that boundary and the framerate goes
in the tank, a large amount of the compute load shifts
from the one higher utilized core to one of the lower
utilized cores and stays there until the "event"
(whatever it is) eventually gets resolved (takes >30
seconds, up to a few minutes to resolve itself).

HOWEVER, should I hit ESC to get to the "return to game/
save & exit" screen, the framerate resolves almost
immediately (within a couple of F3 screen updates).

What I expected to happen was...:
answered above

What actually happened was...:
answered above

Steps to Reproduce:
I have not been able to reproduce this bug EXCEPT in
this one particular world... and in this one particular
spot on the map... but, I'm still looking at it when the
mood strikes me...