RO Unit Needs Some Kind of Help...

Unless your water pressure has radically changed, the problem obviously lies with the unit. If you were happy until recently, then there's no reason to buy an expensive booster pump. Assuming that none of the new stuff is defective, there's only one more component that can degrade over time. If you remove the fitting from the waste line on the membrane housing there's a little piece of plastic in line known as a flow restrictor. It's probably partially clogged with mineral deposits. A new one can be purchased for a couple bucks. It's too bad you didn't post sooner. Had you not purchased a new membrane already you could have easily and for very little extra cost done a serious upgrade in your unit's capacity. Essentially the membrane and the flow restrictor work as a team and must be matched to each other in terms of flow. You could have purchased a 60, 75, or even 100 gpd membrane for not much more than a 24 gpd unit. Two bucks more for the proper flow restrictor and you would now have a much bigger unit. Next time!

I'd replace the lines on the RO unit. Intake and waste may have mineral build up inside.
Or, if you have galvanized supply lines, that line may have mineralization build up. When I replaced my folks galvanized lines with PCV, the galvanized was all but plugged.

Install some monitoring devices..
I have a gauge that shows me the Membrane pressure, and a gauge that shows me the input pressure at the pre filters.
Comes in handy when trying to diagnose an issue.

If you have an adjustable flow restrictor on your waste line, you can crank it down till its barely a trickle. this will increase pressure inthe membrane housing and should up your output somewhat.

Installing a pressure gauge to get an accurate idea of your inlet water pressure is a good place to start. 40 psi can feel like good pressure in a sink or shower but it's too low to get good water flow from an RO. You want at least minimum of 45 psi and best is around 60-75 psi. Once you have that knowledge you can then look at whether there is an issue with the flow restrictor or whether you need to use a booster pump. Subrosa is correct, you can get a lot more output out of your RO by simply changing the flow restrictor and membrane if you want to.

Andy