separate the top faces from the wall faces isn´t killing the FPS.. but it´s killing that ugly cell-Shadow you have ^^ (it´ll look like 1 plane if you do that).

I don´t know how many entities you´ll use for a Map (my maximum is 250x250 and i just show those which are in the view range).. But my FPS are under 40 ._.

My actual testmap:


And the result on FPS:


It´s a grey screen cause there´s no skin on the blocks.. This is the maximum you can zoom out. it shows 1323 Entities.. and the FPS decreases from 500+ to 120-
That´s bad ._. cause i need monsters, rooms..etc to show too..
And that are just the Models in Vew range.. all others are invisible..


My Blocks are using separated walls and top-planes.. means.. each side is a single object in the model... so i´ve no cell-shading on walls, floors and tops
(screen is from my Entity project..)