While it is not a direct answer to your question, I suggest that you do a TDS and/or hardness test on the output water. That should answer the question.
What if you put a higher throughput membrane in 3 months ago, but did not notice the higher throughput until later?
TDS meters can be very cheap. You really should get one.
While it is not calibrated for the purpose, you could use your ohmmeter to compare water samples from your RO and your input water as a cruder test. The input water should be much more conductive. Be careful to use the same probe depth and distance for taking your readings.
You, or somebody in your group of friends, maybe should have a hardness test kit too. It is useful to monitor the effectiveness of your water softener if you have one. It is useful to see if you should have one if you don't. The Hach 5-B is well-respected. If you do have a water softener, the RO system should be fed with softened water.