I'm confident I've been able to found the root cause, no fix yet.

It's easy to reproduce: start new instance, grab resize handle and 
resize away.

I'm looking right now at a trace 23.3 seconds long trace. It has:
- "redisplay" 644 420 times allocating (but not leaking)
- total transient allocation vokume is 1.6 GB
- when removing windowWillResize/setFrame/layoutSublayersOfLayer there 
are only 1765 extra allocations

I think that overall slowness comes from that and fragmentation that 
comes with it. After some time I can imagine it's difficult to grab a 
2KiB slot from OS.

The outliers seems to be drawing glyphs, but I wouldn't focus on it 
first.

IMO there are two root causes:
- Major one: processing all the events without filtering (I have 5397 
attemptResizeWithEvent events handled in 10 seconds - I'm sure I didn't 
resize windows for that long)
- Minor one:  inefficient redrawing, but given number of allocations I 
think that autorelease pool might handle it (once it stops processing 
100MiB/second event streams ;-))

I'll try do deliver some prototype, but I'm not sure if that'd be 
anything integration-worthy.

Best,
Przemysław Alexander Kamiński

---

Most allocation-heavy stacktrace tail (count is total allocations in the 
inspected, 10s long frame):

2	172166  ns_draw_glyph_string_foreground 
[inlined]	emacs	emacs/src/nsterm.m:4278	
1	172166  macfont_draw	emacs	emacs/src/macfont.m:2951	


Transient allocations trace excerpt:

1878.09 MB     100.0%	1613815	 	emacs	-[EmacsApp run]
1874.40 MB      99.8%	1576278	 	emacs	 -[EmacsApp sendEvent:]
1653.78 MB      88.0%	644420	 	emacs	  -[EmacsView 
layoutSublayersOfLayer:]
   24.48 MB       1.3%	184083	 	emacs	  -[EmacsWindow setFrame:display:]
   44.38 MB       2.3%	143554	 	emacs	  -[EmacsView 
windowWillResize:toSize:]
  200.22 KB       0.0%	2237	 	emacs	  -[EmacsView mouseMoved:]
   17.88 KB       0.0%	520	 	emacs	  -[EmacsWindow 
constrainFrameRect:toScreen:]
    7.72 KB       0.0%	494	 	emacs	  -[EmacsLayer display]
   25.00 KB       0.0%	480	 	emacs	  -[EmacsApp stop:]
   77.17 KB       0.0%	177	 	emacs	  -[EmacsView viewDidEndLiveResize]
   14.62 KB       0.0%	78	 	emacs	  ns_send_appdefined
   20.58 KB       0.0%	68	 	emacs	  -[EmacsView windowDidResignKey:]
   14.09 KB       0.0%	213	 	emacs	 -[EmacsLayer display]
    5.73 KB       0.0%	65	 	emacs	 -[EmacsWindow 
accessibilityAttributeValue:]
     192 Bytes    0.0%	1	 	emacs	 ns_send_appdefined