Haha the speed button has received lots of love, understandably so!
And rules are definitely unclear in-game, so that's my bad (ran out of time to add a little "how to" or tutorial).
The aim is to build apartments that have at least one bedroom, bathroom, kitchen, and living room in them, and you then boost multipliers by having as big an apartment as possible (fill up the row) and any double+ sized rooms (same icons next to each other). Penalties if your apartment is missing one of the rooms (icons). There's a bit of an illustrated guide on the game page which might make more sense.
I would've loved to do something more with the score. Originally wanted more levels (different building layouts) you could unlock if you met the property value target, plus a leaderboard for each. I plan to add those to a future full launch of the game.