The flow restrictor is located in the black tubing, the waste line that is connected to the end of the membrane housing. It restricts how much water exits the unit, creating sufficient back pressure so the RO membrane can do its job. It is possible for the restrictor to clog or be pinched, but this is very rare. If you remove the black tubing for any reason, be sure you keep track of this little part. Below are several pictures of it, so you can see what it looks like and how far is should be installed in the tubing. Depending on the age of your particular unit, the flow restrictor may differ in color or design:
The flow restrictor must always match the membrane or production rates may vary greatly.
If you have this flow restrictor (pictured below), it should be spliced into the black tubing waste line, with the arrow pointing toward the receiving drain. Any other flow restictor should be removed from the system first, as this is the replacement.