For given physical filter size, a finer mesh = better filtering, but at the cost of lower flow (gallons/minute). You apparently bought a replacement that filters better, but your flow rate dropped. If you want to get the flow back, choose a different filter (larger micron value). Something like 20 microns (the size of the pores in the filter material) will pass a lot more water than10 micron, for example.
Ditto on the suggestion for rvwaterfilter.com. You can get the exact filter product you need at a reasonable price, plus the site explains the tradeoffs.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC