1. A computer implemented method of conserving processing resources used to render content pages, the method comprising: as implemented by one or more computing devices configured with specific computer-executable instructions, identifying a plurality of tiles of a content page requested by a user device; identifying an off-screen tile among the plurality of tiles, the off-screen tile corresponding to a portion of the content page outside a visible area of the user device; for the identified off-screen tile, calculating a probability that the off-screen tile will become visible; calculating a measure of an amount of computing resources needed to update the off-screen tile; selecting a timing with which to update the off-screen tile based at least partly on the probability that the tile will become visible and the measure of the amount of computing resources; and updating the off-screen tile based on the selected timing.