Allocations would get fast ram with priority, unless chip ram was specifically requested.
Fast ram is exclusive to the cpu, and unaffected by the chipset accessing chip ram.
Amiga had this fast ram concept. Atari ST unfortunately did not.
Atari TT did. And is possible on Falcon with expansion.
The main issue is that the OS did not have a way to request "chip RAM", thus software compatibility hell if added too late (never actually got added AIUI).
I think Atari Corp had a burst of engineering competence at the end after Tramiel Sr handed over the reigns to his sons. But it was too late, the market moved on.
The Falcon is a lovely machine. The last TOS releases addressed some longstanding problems. Hiring Eric Smith to work on TOS/MuliTOS was a good, but late effort. Porting SystemV and trying to make a budget Unix workstation was a doomed but clever effort (I still remember the UnixWorld news clip box headline "Up from toyland").
I bought a 486 late 1992, put Linux 0.97 on it, and moved on.
Realize how late that is, relative to ST's release.
While it was kind of a crappy games machine compared to the Amiga, it certainly was better than a PC or a Mac, and as an all around general home productivity computer was the best value for the $ out there at the time. Which I think was the Tramiel's aim.
But the Atari ST didn't have custom chips, so there was no need for chip ram/fast ram?
The problem persisted through the Falcon, which 68030's was tremendously held back by lack of a fast ram concept.