Was it a 75 gpd membrane you took out?
Connected to the faucet, you take out the water saver washer in the faucet?
Or Saddle tap valve on sink supply water?
600 or 750 flow restrictor?
In line? Was the old flow restrictor in line?
Does flow restrictor have an arrow pointed the right way?
Flow restrictor installed on waste water line?
How's the flow coming out of waste water?
You may have went from a 5 or 10 micron particle filter and/or carbon block down to a smaller one micron, and /or a 96% rejection rate membrane to a 99%. While these things will effect your production rate, the difference wouldn't be night and day. My first guess would be defective, miss sized, or mis placed flow restrictor. You could get a jug and time/measure your restrictor. As an example, a 500ml flow restrictor should give you around 500ml through that restrictor an hour. Or you could take your old one and install it to see if that fixes your issue.
To get decent life out of your membrane you should be at around or above 4 to 1 ratio of waste to product water.