don't understand these results - https://youtu.be/1hczR5gxhSc
it's even slower then common pipe line with common pumps...

there needs to be another intake or outtake after every underground pipe or the fluid just seems very slow

the pipes would work alot better if the fluid amount in them were higher, maybe a buff from 800 to 3200 or even 6400 per piece, and make them a bit more expensive because of the buff

a pump piece says it does 72000/s which would mean 1200 units per game tick, since a second has 60 ticks,
BUT the pipe is only capable of storing 800 fluid amount, so the pump will never work at full speed even if the pipe is 100% full every tick of the game

yea, smaller pipes seem to be more of a bottleneck

even the guide of this mod shows that intake/outtake after every section is bad way
https://i.imgur.com/Cd1dnkU.png

but if you build like that guide tells, it is really really slow flow, kind of defeats the purpose of having these big ducts :S

guess there is just no way for a really fast pipe with the current fluid algorithm of the game

Technically, this is not a pipe in terms of game logic. And to increase the throughput you need to use as few sections as possible. Like that:
https://ibb.co/kXdgFcb

so in conclusion - we receive speed increase only if we use 1-2 sections - all other scenarios lost to common pipes (even scenarios that we see in description screenshots)... i think this conclusion must be 1st line marked with red bold font in mod description...

Look this image:
https://i.imgur.com/Z71grSu.png
How u can see the first is wrong, but why? The 1x2 section have a fluidbox of 200 unit, this mean that is the double of one pipe, BUT in ur example (that is the same case that in the picture I mark as wrong usage) u got a problem that u can look deep in this discussion:
https://mods.factorio.com/mod/FluidMustFlow/discussion/5d7d5fc5e18d8c000bb33139
Summary, for a full throughput u need that the entity have a full fluidbox but because ur input is of max of 200 unit (the small duct) u have create a bottle neck and the underground duct is of 800 unit, so the real throughput is 25% of max throughput so is one pipe, meanwhile u have use the steel pipe that have 125 unit but will be full so they will have max throughput of 125 that is more than one pipe.