The ProgressBar is a 1×1 tilegrid configured to display a W×H bitmap. Instead, it could be a W×1 tile grid, displaying 1×H slices from a 3×H bitmap.

Three tall elements are needed: One which is entirely the outline color; one which is the outline color at the top and bottom and transparent/black in the middle, and one that is the outline color at the top and bottom and the bar color in the middle.

This should reduce memory consumption (from W×H to W+3×H times some constant) and speed updates, since a progress change P pixels wide would only have to update P tilegrid entries instead of P×H bitmap entries. (however, the amount of data actually sent over SPI/I2C to the display would not change)