Seems that when Ultimate Belts are used against Factorissimo buildings, the belts contents are not going through smoothly at all.
As an example: I placed a MK3 Factorissimo Building, using Ultimate Transport Belt (Cyan 240 items) the highest, and once my items hits the wall to go through 2 and 2 items will slip through every 5-6 seconds.
It will however completely stop in between.
This happens on the way OUT of the building as well.

Not entirely sure if this is due to limitations in Factorissimo's buildings or what it is.
Granted, base game belts works fine obviously.
Should be mentioned that the "Ultra Fast Belt" of yours, the 80 items/s works fine against the building.

It's most likely based on item speed against the wall ports.
Just wanted to let you know!

Thanks for the report! I just had a discussion with the mod author, MagmaMcFry, on discord and he will be working on a fix soon! The problem is with some array calculations Factorissimo2 uses to transfer items on belts. He is going to update the array calculations so that they can handle the belts that are faster than 120 items/second. So pay attention for Factorissimo2 updates, hopefully the next update will have the fix :)

tested with latest version as of today and the problem still seems to prevail, any update? thx

That seems to still be on the Factorissimo side.
:/

Right, still the same problem also in my game.

i am having this problem too, any solution, or anywhere to report it to ??

Hi, idea for test workaround:
try use some chests for buffer (long time ago it work fine with bots) + loader/miniloader mod for fast filling and emptying.
Set chest as one-way, not balance.
Don't know how UPS friendly it is, and it may not be good for too mixed lines, but for pure belt lines speed should be fine.

Wonderful suggestion! Honestly I think I recall that being more UPS friendly than the direct belt connections, but I'm not 100% certain. It's beyond my coding ability to add the needed support, but you could request the support for higher speed belts here: https://mods.factorio.com/mod/Factorissimo2/discussion