How to make water filters last longer?

I did a bunch of research into benchtop water filters and figured that one that would hold 4 filter cartridges would be big enough for our needs.

At first it was excellent, but after a few months the flow rate slowed down massively.

I am finding that when I scrub all the filters and replace the water with fresh water, the flow rate is a bit better, but if I leave it longer than a couple of days without scrubbing or replacing, it slows down again.

When we first got the filter, the water level in the top bit would drop down almost to the bottom, now it only empties to around 3/4 of the way up the filters, so overnight it just doesn't refill like it used to, and during the day it's definitely slower than it used to be, so I'm having to constantly top it up during the day, and am not sure if it's going to give us enough water this summer.

I got the Boroux filters, because I did some calculations and figured I would only need to replace them every 10 years, whereas other filters I looked at needed replacing every year. Is the slowing down of the filter a sign that it needs replacing? Or is it still filtering properly, just more slowly?

Does anyone have any tips for getting filters to last longer? Or any DIY ideas for low pressure off-grid creek water filtering?

It sounds like biofouling. Perhaps these filters are designed for already -filtered municipal water? Most are.

When I scope out a reverse osmosis system from our well water, many many layers of filtration are required before they get anywhere near the RO part of the system. It's a royal pain.