Flowing liquids are hell for your FPS as well, particularly magma because it creates temperature recalculations.
Not really. Only poorly built magma pump stacks, and perhaps magmafalls as well, will cause temperature recalculations.
Temperature recalculations do not happen that often at all with flowing magma.
Only when a tile adjecent to magma loses all magma neighbors, or a tile that was not adjecent to magma gains a magma neighbor, will it's temperature need to be recalculated.
1/7 magma still heats neighboring tiles. So a pool with 5/7 magma wobbling about will not cause more fps drain that that same pool with 5/7 water.
Practical example with pump stacks:
I will try to explain while this pumpstack configuration will not cause temperature recalculations:
wwwww wwwww w - wall
ww.ww w+++w . - channel (pump intake)
wwVww ww^ww + - floor
wwVww ww^ww V ^
w+++w ww.ww V, ^ - pump (from north, from south)
wwwww wwwww
z(0) z(1)
While this one does:
wwwww wwwww
ww.ww ww+ww
wwVww ww^ww
wwVww ww^ww
ww+ww ww.ww
wwwww wwwww
z(0) z(1)
Let's start with the bad pumpstack, and see what happens over time, when it actually pumps magma.
wwwww wwwww
ww.ww ww+ww 7 - 7/7 magma
wwVww ww^ww
wwVww ww^ww
ww7ww ww.ww
wwwww wwwww
z(0) z(1)
at t=0, we have some magma in the bottom pump's output reservoir, heating up the walls around it, and the part of the pump it touches.
wwwww wwwww
ww.ww ww7ww
wwVww ww^ww
wwVww ww^ww
ww+ww ww.ww
wwwww wwwww
z(0) z(1)
at t=1, the pump sucked up the 7/7 magma from the bottom level, causing it's surrounding tiles to recalculate temperature, and the magma now heats the tiles around the upper pump level's output reservoir. These states will keep switching, causing temperature recalculations on every frame the pumps are actively filling something. They will eventually stabilize once whatever it is you're filling is full, and no more new magma needs to be sucked through. FPS pain though, when activating a larger pumpstack made of these elements.
Now the other design. Why does this not drain FPS?
wwwww wwwww
ww.ww w+++w
wwVww ww^ww
wwVww ww^ww
w777w ww.ww
wwwww wwwww
z(0) z(1)
at t=o, the tiles surrounding the bottom pump's output reservoir are all heated.wwwww wwwww
ww.ww w+7+w 1 - 1/7 magma
wwVww ww^ww
wwVww ww^ww
w1.1w ww.ww
wwwww wwwww
z(0) z(1)
at t=1, the tiles surrounding the bottom pump's output reservoir are still heated, thanks to the 1/7 magma left on both sides of the center, from which the pump above draws.
There will be no more effect on FPS once this pump stack is running, it will not need any recalculations after first initialization.
EDIT: crap, colours don't work together with code tags
I had painted it so nicely maroon and red for you.