My water runs around 40 psi too and it takes 4 hrs to fill a 5 gallon jug. Are you running your RO
directly into the tank or into smaller containers that you then empty into the tank? I run water into the 5 gallon jugs if it's during the day and into my 20 gallon rubbermaid if it's overnight. You'll need some sort of large container (ideally 20% your total volume) for the future to always have PWC
water ready and waiting. If it's big enough you can run it overnight without worrying about overflowing. I've done that a few times...what a mess.
I agree with the suggestion about adding the salt to the sump, not the main tank, but that only works if your system is full of water so you can turn on the pumps and heater. If you can run the RO
overnight you may get the system filled sooner?
It's only this hard the first time around - from now on you'll only be doing partial changes! And this is your best opportunity to start out with the best possible water conditions. A couple more days - Hang in there!