Plotting SP needs to be optimized.

[mrecachinas]

Describe the bug Plotting scalar packed files on larger (~15 MB) files (i.e., overlay_href) is currently inefficient. Changing SP to SB on the server-side, while space-inefficient, speeds up plotting. Also, plotting SP blocks the entire page, so if you have multiple plots and the SP overlay_href triggers first, none of the other plots will render until SP is done.