Quick take
Check whether the system is tankless or tanked, then inspect filter age, feed pressure and membrane production before replacing major parts.
Clogged prefilters
Sediment and carbon prefilters protect the membrane. As they load, feed pressure to the membrane falls and production slows.
Low feed pressure
RO performance depends on pressure across the membrane. Low household pressure can reduce production dramatically.
Storage tank problems
On tanked systems, incorrect air precharge or a failed bladder can create weak faucet flow even when the membrane still produces water normally.
When a newer system makes sense
Modern tankless systems from Frizzlife, Waterdrop and others offer much higher published flow, but proprietary filters and electricity are the tradeoffs.