No need for assumptions 

 the test environment was outlined in 
https://forums.macrumors.com/posts/23128071 above.
Technical
	
	
More than just defragmentation of 
unchanged data/metadata files.
For my most recent round of testing, I allowed 
significant changes to two large and essential metadata files: the catalog B-tree and the attributes B-tree. Such files are normally written very close to the start of the (HFS Plus) volume and:
- where the volume is on rotational media, that closeness goes towards optimal performance
 
- with or without rotational media, contiguity of both files goes towards optimal performance.
 
For this round, I allowed iDefrag to make the required changes to those two files.
When I last checked, iDefrag 4.x made use of 
fsck_hfs(8) for each rebuild. One rebuild (option -R) with flag 
a, another rebuild with flag 
c.
A recent view of the layout, showing an attributes file that is relatively small (less than 1 GB):
 
	
		
	
	
From that article it seems to me that Lewis Painter lacks some breadth and depth of experience.
Less technically
For pre-release Sierra, I'm performing very lightweight tests in an unusually 
constrained environment.
In the past, I more often performed 
heavyweight tests in a good or better-than-average environment.
Either way: there's likelihood of 
reproducibly exposing a symptom (maybe a bug) that might never be found with, say, an all-SSD environment.
The greater the exposure before release of the operating system, the easier it should be for Apple to address issues in good time.