A few responses to me, so I'll deal with them like this:
Quallada: I am not exactly why would Flash not give everything he has in a fight like this. There is no point assuming he would move slow enough for Deathstroke to trip him in this bout. Otherwise we could just say Squirrel Girl won Thanos and call it a day. Also, note that I am not saying Runner is weaker than Flash, I am saying that IMP is all Flash needs.
Lord Feron: Truly?
Juntai: Correct me if my memory fails here, but didn't Runner just beat the living daylights out of Thanos at first?
Bouboumaster: Doesn't really matter at those speeds.
Because 1) CIS applies, 2) Thanos may not be Speedy Gonzalez himself, but has been shown (Runner aside) to be incredibly good at repelling blitzes, 3) Ignoring Flash's lower feats is a selection bias (despite the obvious PIS implications at times) and 4) Thanos has handled much more than an IMP.
Personally, I dislike debating about Flash, because a lot of what is bandied around is based on hypothetical levels that we will never see on panel partially due to story-restrictions. I can't assume a certain level just based on the possibility of said levels after extrapolation.
What Morrison did with the speed force and IMPs cherrypicked a portion of Einstein's theory while doing away with the other via the speed force catchall answer, though.
I am more curious about how any character without cosmic senses can see properly post-c as light bouncing off objects cannot travel as fast as the speedster. Clock one more to comic book science then.
I don't think Speed Stealing will work on Thanos thru his shields. I'm pretty sure that he's considered shielding himself from energy/matter manipulators.
Also, LOL at flash being able to hurt Thanos. SS blows up planets like they're nothing and Thanos has been able to tank an enraged Surfer's blasts at point blank range and was completely unaffected by them. This is WITHOUT his shields BEFORE his upgrade. No way is IMP even going to affect Thanos' shields.