[css-grid-3] Remove outdated performance notes

Author: fantasai

css-grid-3/Overview.bs

@@ -1419,30 +1419,6 @@ Absolute Positioning</h2>
 	Maybe it could defined as the max (or min?) current [=running position=]
 	of the [=grid-axis=] tracks at that point?  Or the end of the item before it?
 
-<h2 id="performance-notes">
-Performance Notes</h2>
-
-	In general, masonry layout should have significantly better performance
-	than the equivalent regular (2-axis) grid layout,
-	particularly when the [=stacking axis=] is the [=block axis=]
-	since the intrinsic sizing of grid rows is typically quite expensive.
-	Any intrinsic track sizing in the [=grid axis=] should be cheaper too,
-	because, typically, only a subset of items contribute to the intrinsic sizing in a masonry layout,
-	contrary to a 2-axis grid where all items spanning an intrinsically-sized track contribute.
-	Stretched items do a second layout with the new size (when it actually changed)
-	so this can be costly if there are a huge amount of stretched items
-	that each contains a lot of content.
-	Especially nested stretched masonry layouts should be avoided
-	unless they are small/trivial.
-
-	Advisement: This can be ameliorated by the author
-	by opting out from the stretching on most items though,
-	e.g. specifying ''justify/align-items:start''
-	and then opting in for just a few items with ''justify/align-self:stretch''
-	to let those items fill the [=stacking axis=].
-	(This performance analysis is from a Gecko perspective,
-	but I suspect there's some truth to it for other layout engines as well.)
-
 <h2 id="graceful-degradation">
 Graceful Degradation</h2>