The OEM Motorcraft FL820-S has a flow rate of 2.62 gpm with 75 psi @ 2000 rpm (~ 92% efficient @ 20 microns). Visit "Bob is the oil guy.com" for details like this.
The cleanable, full flow oil filter replacement has a flow rate of 20+ gpm and "Filtration: SAE J1858 Tested: 90% more efficient than "throw away" filters."
The by-pass filter is flowing a mere 1/8 of the flow rate of the full flow filter (not a factor in the overall scheme of flow rates). It's job is to remove the smaller particles and the "dirty" part of oil. This is the part of this set-up that extends the oil replacement intervals.
Finally, the only restriction in this set-up: the 1/2" ID hosing between the adaptor and the filter bracket. Typical flow rate for 1/2" ID hose at 75 psi is 22 gpm (minimum bends; run lengths less than 30").
Amsoil, the leader in the area of oil by-pass systems, follows this same path of flow management and resulting flow rates.
The goal of this "fun" was to minimize oil changes, and make the effort of changing the oil as painless as possible. This setup does both, without breaking the bank.