See https://forums.factorio.com/viewtopic.php?f=49&t=70896
Having half of base in Factorissimo factories drops UPS by 2 times. 100 electric networks are 5 times slower than 1 of same total size (10 in that case, but must be considered that half of base is within them), also the Factorissimo script takes some time. This makes ABPy game unbeatable without UPS drops on very top CPUs. Setting electric updates to each 5th only influences the Factorissimo script update time, but not the electric networks. Can anything be done?
Also, where is the Factorissimo thread on the forum? Can't find it.

Each factory building needs its own electric network, because you can't connect electric networks on different surfaces. So the only thing I could do about electric network performance being bad when using Factorissimo is to petition the devs to add some way to connect electric networks across surfaces natively, then make Factorissimo use that if/when this becomes implemented.

I have one more suggestion that might help: Check that all your factory buildings are set to transfer enough power for their interiors. I've heard that electric networks are much more optimized for the case that they're fully powered.

Also there's no dedicated Factorissimo thread on the Factorio forums.

Would be nice if you petitioned them. :P I tried in that thread and failed. For a different thing though.