We do not throttle RAM usage. If you got a performance increase from replacing the RAM, it’s because the RAM got faster, or you are experiencing a placebo effect.
If you have too little RAM, Roon does not use slower storage to keep working, it just runs out and crashes.
Some systems (windows, macos, many linux distributions, etc…), depending on the install, will swap out the contents of less commonly used physical RAM to SSD/HDD. On those systems, you would feel a massive performance drop if you run out of physical RAM, and increasing RAM would prevent that drop. However, Roon OS does not do this. If you run out, you will crash.