Not a bug. In the city you're looking at how it's also affecting upkeep and how that tallies in.
On left side at buildings screen, its -15, uncounted standard display but pedia says -10, so pedia should be set to -15. I dont think the left buildings screen in city displays the affecting upkeep, only the standard cost.
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Its the base cost, same in all citys, i mark it red for you into the screenshots look it up, -15 base pedia says -10 so courthouse should be -10 or the pedia changed to -15. (Or is this because of deity difficulty lvl? But then the pedia should adapt.)
You can make the single core game use all your cores and make it faster, here is how you make your game faster as i show in my stellaris map mod Ultimate-Galaxy: