gristHelper.Display are real columns.
It is used for Reference and RefList columns. For example, if you have 3 reference/RefList columns in your table, you’ll see gristHelper.Display / gristHelper.Display2 / gristHelper.Display3 when you check formula performances.
These columns have an impact on performances, but I’m not sure if they make tables slower all the time or only when you want to update these values.
It seems that performance, as visible in the Grist Formula Timer, is linked to the size (and speed) of the table linked. If your reference is with a big table, it’ll be visible in the Grist Formula Timer.