Philosophía
"The devil made me do it"
The thing is, forum fights aren't comic book fights. That's why we're removing the PIS part - plot induced stupidity. The threads here have no plot, and we're using the characters to the best of their intelligence and character. Being fast is just one of those things that happens to be so basic, the characters would have to be mentally impaired not to dodge to the best of their ability if they have [as forum rules state] common knowledge of the opponent.
You're right that it doesn't take much more than a small speed advantage to make your opponent look like a fool, but many here can't wrap their head around that. I always think going to a mma fight video on youtube, slowing it down to 0.25x, and imagining that only a million times slower, and then Superman/Flash being a million times faster than even THAT should drive the point home. But even that's hard to think about, since there's no point of reference, and after like 0.01x the fighters in the video are a statue either way. I don't want to turn this in 'what does that mean if Superman fights that character?' because it would just keep going, but if you can warp reality [like celestials scan], time/distance [i.e. speed] isn't much of a problem. And even then, see: Flash vs Anti-Monitor. It's not unprecedented.
As for your Thor question - travel speed doesn't translate to reaction/combat speed. The same way it's hard to wrap one's head around how fast being that fast is, it's also hard to get perspective on how big space is. If I give you now, a normal human, the ability to fly at 10 times the speed of light, and tell you to fly out of the solar system, you'd 100% not hit anything. Space is so empty, that it's ridiculously impossible to hit anything, unless you really want to. Think of the fact that ALL the planets in the solar system fit between Earth and the moon, and now think how big the solar system is. You apply that to the galaxy, and it's like trying to hit a specific speck of dust while you're jogging. Same applies to Green Lanterns historically, who by and large are normal humans crossing the Universe.
Any time somebody says that some posters are wanking Superman [i.e. 'Wah he beats my favorite character!'] I always say that you have no idea how unbeatable a wanked Superman would look. 'Almost as fast as Flash' translates into 'basically the Flash' for any thread of him not facing the Flash himself, and when we really start wanking his other attributes, well, that would drive people off the forum, probably. But that would be no fun, so that's why you see Superman vs Thor [or insert any other character with no high-end superspeed] as basically Superman fans underselling it, not going to the true extent of the implications beyond 'he's faster' and focusing on other stuff, because most threads would end at the first post, and it would ruin the fun of getting and debating other stuff [at least lately, minus h1]. That's why I'm a always telling you that Kal-El squad will welcome you with open arms, you have no idea how fun it is to hold back arguments yet still be able to easily argue your case. 😛