Those are probably fairly common in image compression standards - designers of image compression are not, in general, going to make their compression less efficient than it has to be. JPEG2000 in particular is likely to be far worse.
Not at all. Decode performance is generally a very important consideration, as it has to go in all kinds of devices. Unbounded decoding time would be terrible (it would "lock up" embedded devices), and it's almost certainly an oversight.
You probably have encoding performance in mind, where there's much more liberty to make things very heavy.
No, the opposite is true. Fast decompression is usually either the first or second priority for images.
However, more interesting were the two medium vulnerabilities, which were initially reported as DoS bugs in the libjpeg-turbo library but on further investigation were found to be issues with the JPEG standard itself. These issues were reproduced across multiple JPEG implementations, can be triggered by entirely legal JPEGs, and so are not easy to mitigate in any JPEG library itself.