To see timing of calls to texSubImage2D with canvas as the data source, open up the console. On newer Chrome with accelerated canvas, times often go up to 2 or 3 or as high as 6. With previous versions of Chrome that did operations like text rendering in software, the time is consistently 1 ms max. The disparity gets worse as the GPU pipeline gets more and more full. In an app that really taxes the GPU, the times can be tens of msec.