Est. Performance Impact is a now removed UI element from the Open and Closed Beta versions of TABS it was shown near the bottom of the screen, dictating approximately how much lag the number of units you have placed will cause. The higher the Impact is, the more likely the game will crash or lag out.
Est. Performance levels go in the following order: Light (0 units), Medium (50 units), Heavy (100 units), Very Heavy (150 units), Super Heavy (250 units), Incredibly Heavy (350 units) and as a kind of joke, the last one is called Stahp (500 units). But as you add more units, the "Stahp" gets longer, by adding A's into it, until it reads "Staaaaaaaaaahp" with 10 "a"s. If even that is somehow exceeded, it will simply read "404" as a joke. In the Pre-Alpha version, it only went to Incredibly Heavy. This feature is not current build of the game, as it was removed after the Open Alpha.