-Yeah currently in the late game demodwarves scale the most because of underground mapping infinitely buffing them so much. In the full game every unit will have a similarly powerful way to scale. Eg some may benefit from shrine or meadhall or wizard towers or just other normal upgrades/artifacts yet not added.
-The incorrect cost display was an issue few patches ago where the display was different to the effect of scientists. Were you perhaps playing on an older version?
-Yeah the upgrade panels aren't fully optimized yet and can lag when buying upgrades in the late game. Though if it happened to you early it's a bit stranger.
-The performance is a tough nut to crack, especially since the physics engine can hardly be optimized further. There is still work to be done in other areas such as falling rocks, though reducing their count isn't as simple to make work.
Thanks for the feedback! :)