Using Efren as an example is probably not helping your side of the argument. He literally won his very first event on U.S. soil. He also came from the Philippines (which I know you're well aware). Do you really think he wasn't familiar with and dominating the competition on "slower" tables back home? His biggest problem early on was just getting over the finish line, he wasn't having trouble going deep in tournaments.
It's not that I don't believe their accounts, it's that the archived video evidence continues to accumulate, and most of the video evidence, probably at least 9 out of 10 videos, if viewed through an objective lens, doesn't show these really slow conditions that many would like us to believe. The IPT experiment wasn't that long ago either and it just didn't prove out that the slower cloth changed things that much. Efren (see above paragraph) didn't have any trouble, and the tables didn't even really appear that slow anyway.
Some of this is probably due to the "back in my day" narrative that we can all be guilty of from time to time.
If I can find it -- I think it was CreeDo who had a great post about fast cloth years ago where he spelled out why he thought it was more difficult. I'll see if I can find it.
During the whole transition to Simonis there was the period where the players complained about things getting too fast. I think Greg Sullivan played a role in the whole transition. I think they tried 760 first and then finally settled on creating a new cloth -- 860. 860 isn't even super fast in my estimation. It all has way more to do with the rails. If the rails are super bouncy, then it doesn't really even matter what cloth is on the table. This is what we see repeatedly in old videos. The healthy tables with lively rails and the older woolen cloth appears to perform just fine. Contrast that with even more recent videos you can find of matches on slow tables. You'll sometimes see this with older Gold Crowns, that are either in need of cushion replacement or they were recently recovered and probably have the rail cloth too tight and needing to be broken in. If the cloth is fast and the rails are slow -- good luck getting the ball around the table.
Bottom line for me -- I think the older woolen "slower" cloth gets way too much focus. Video after video show fast tables pre-Simonis. So maybe, just maybe the culprit of slow tables, where they existed, was more attributable to poor cushions.