Exec this code:
? 'Direct mode'
DEFINE POPUP pptest relative
FOR k=0 TO 20
t1=SECONDS()
FOR IJ=k*1000+1 TO (k+1)*1000
DEFINE BAR m.ij OF pptest PROMPT '\-'
NEXT
? ij-1,SECONDS()-T1
next
RELEASE POPUPS pptest
? 'Reverse mode'
DEFINE POPUP pptest relative
FOR k=0 TO 20
t1=SECONDS()
FOR IJ=k*1000+1 TO (k+1)*1000
DEFINE BAR m.ij OF pptest PROMPT '\-' BEFORE _MFIRST
NEXT
? ij-1,SECONDS()-T1
next
RELEASE POPUPS pptest

I do not know the inner structure with which the "relative popup" are implemented.
In the hypothesis that was used of the Array, I expected that in reverse mode the construction it was slower.
Until 6000 elements, it is therefore (slower 300%).

Surprise, beyond the 6000 elements (out of cpu cache memory mapping) the duplicated search of the id is 50% more express.

Deduction: it is not implemented like Array, but like list, and the direct algorithm is not done good.

Any comment?

Fabio