From the comments it sounds like the onboard pump may be very slow. I don't use the PTO at all and it's not too late in my build to make a tower the reservoir.
I'm not sure I would characterize it as "very slow". I have a very similar B7100. Like your machine, the pump is a 3gpm pump. I built my FEL with 2" diameter cylinders, 16" long, powered by the on-board hydraulics. 4 cycles of the lift arms from bottom to top and back down again took 63 seconds -- just a hair under 16 seconds per cycle.
It turns out 2" cylinders were overkill. They can lift far more than the front axle of the tractor can handle without risk (can lift roughly 1,900 lbs at 10° angle at the pins). 1-1/2" diameter cylinders would have been more appropriate (can lift almost 1,100 lbs at 10° angle) to the capacity of the tractor (500 lbs material + weight of FEL itself).
Had I used 1-1/2" diameter cylinders, the calculate cycle speed would have been about 9 seconds, which is quite respectable. Note that is based on an empty bucket, I would expect the time to lift a full bucket would be longer, particularly for a given weight -- the increased speed is at the expense of greater load on the engine and could slow it down as things load up. But then, the same is true of a PTO mounted pump -- you don't get anything for free and the engine can only provide so much power no matter how you connect to it.
For the sake of comparison, I ended up replacing the stock 3gpm pump with one rated 6gpm, but only saw a 20% speed improvement (13 sec/cycle). Nothing to write home to Mother about, but quite usable in actual practice. In fact, I find I need to slow the engine speed right down for better control when using the forks.
As for your original question about hose diameter, I think it depends on how long your hoses are. If you are running hard lines for most of the length, with short lengths of hose limited to the actual cylinder connections, then 3/8" hard lines with 1/4" hose is fine.
However, if you are running the whole thing in hose (as I did), then I suggest you increase the main runs from the spool valves to the tee in 3/8", since that section is feeding two cylinders and has to carry twice the volume, plus friction loss and fluid heating is a function of both diameter and length. I ran the T line from the spool valves through 1/2" hose to the transmission sump for the same reason -- at times it could be carrying the volume from 4 cylinders.