DiGiCORE : in fact, what they said isn't very relevant to HTML5. They are working with native binaries. When going native, XCode on iOS is using a fileformat that is indeed having the .png extension, but in fact it's not a true PNG (for efficiency reason, Apple swap internally the channel order of their file, to let OpenGL ES, the backend displaying the images and doing manipulations on them, go faster).
Because of that, if you load true PNG files in an XCode project, without "optimzing" them the Apple way (swapping the channels), the app is going to do it at load time. Meaning you'll be slower than any other fileformat.
As the original author is saying, when you optimize them, and make a pass of PNGCrush or a similar tool on them, they are as fast (or close) to other file format.
Finally, browser (HTML5) don't support natively TGA. You can only go with BMP/JPG/PNG(and XBM/PBM but don't deal with that, it's old, buggy and unuseful).
If you want to load another file format, you need to create a wrapper to deal with that format in JS, and it's going to be slower that the eventual benefit of a "faster" file format...