2 Important things I have to clear up.
Good media - Anything Taiyo Yuden i've come to the conclusion, that's my opinion.
Bad media, anything below Verbatim, even some wiis have problems reading the verbatim media, unless set at 3x or slower. Due to a rumor, the wii disks them selves are supposed to be TY media, therefore the drives will support TY media natively, whereas to other medias have problems, due to almost unsupported media, but wiikey has support for this so no worries, well some support anyways for the medium class media and better.
The Read Speed if for Usage with cheaper media, generally the bad medias won't perform at their unspupported speeds, meaning if you have 4x supported memorex and try to run at 8x, it will sometimes DRE, that's why you should use Better media, because they function close to their recommend speeds and above or below it. The read speed on Wiikey is also to lower your drives' sounds, at 6x it makes a sonic v-roooom noise, when opened during modding, when closed, you can notice it if your room is quiet, whereas to 3x is slower, and is suppose to read better, for the bad media anyways. 3X is noticeably quieter, almost 2/3 quieter where as to 6x. But generally 6x Disks Read faster during loading for good media, if you are using bad media, the load times will always be slow. Also the load time varies and is never the same, it depends on the game.
If I am wrong in anything please correct me, this is from experience and reading dvd media sites like
http://www.videohelp.com/ and others.