It does work on my system now and i still recommend the flat one as you can easily finetune the drawings of subfields (lod)
BTW zooming onto single subfield almost hangs your app on my system. Does it work smooth for you ?

Also as far as i could see you are directly representing the logical sizes as "onscreen" display size.
For eg with chipsize=50000
FieldSize= QSize(32000,32000)
SubFieldSize= QSize(4000,4000)

And you are using this size for each item!

If you need more optimization and if it is possible, do scale down each of these so that on screen "transformation free" size of each item is within (500,500). It is far easier to just multiply the coord by scale factor and then display in sidebar than actually drawing the item in those huge sizes!

Otherwise you can also try to use QStyleOptionGraphicsItem::exposedRect in FieldItem:: paint method and draw only visible part. But i guess this is quite hard and might not work as required.