cool 🙂,
Surfer's level of sub-atomic control is demonstrated in his fight with UniLord (a Skyfather level God) where he is literally sliced and diced and smashed to bits but reforms instantly. he does require outside (rather than internal stores of) Power Cosmic when he's up against high Skyfather+ characters to even stay alive - as demonstrated with the T&A fight, but at herald/sub-herald/top-tier, it's not an issue.
like say Magneto's got total control over magnetism but, up against Surfer, Surfer would own him with magnetism.
the Stranger and other above-herald level characters would own Surfer pretty much as a matter of course. Runner not only out ran Surfer, he over powered him too... it's the way things are, hierarchies. with Flash though, this never can come into consideration.
Furthermore Flash has no access to, no connection with The Power Cosmic, which Stranger et al have (Watchers, Thanos, Galactus being the ultimate wielder of Power Cosmic), which is why they can cut off the Power Cosmic from reaching Surfer. (note, recently Sakaar's wormhole also did that). it's not that Surfer is somehow unable to access it, it's that the Power Cosmic no longer reaches Surfer. the object affected isn't Surfer but the PC itself. 'cos once Surfer left Sakaar's atmosphere he had access to the Power Cosmic again.
I've only seen Galactus shut down Surfer's Power Cosmic, and that is a feat beyond anyone else.
I hope that takes care of both why Surfer can be altered by beings essentially more powerful than himself, whilst still retaining the ability to recreate/regenerate himself eventually.
as to Flash stealing speed, it is not analogous to say that Flash could steal PC, they are vastly different things. Speed is kinetic energy, as far as DC goes. Power Cosmic is that and a whole lot more, consciousness, time, dna... everything in the universe essentially.
thus Surfer can always (and does always, both consciously and unconsciously) generate kinetic energy both internally, via the huge stores of PC in his 'shell' and externally from the cosmos around him.
Flash can't prevent Surfer from accessing this (as Flash can't tamper with Surfer's powers), thus it becomes futile for Flash to even attempt it.
of course ALL this is predicated on Surfer allowing Flash to attempt to speed steal. which he might, out of vindictive cruelty 😄.
as for LENDING speed, what possible "speed" could Flash lend to a creature faster than him, both in mental processing and motion?
surfer doesn't just 'lend', he creates other Surfers, on a whim, equally powerful (because the PC is out in the universe, all he has to do is make a being with his access to the PC) and without losing anything in the process.
Surfer, via matter manipulation, has also given life to creatures... Surfer can also kill someone with but a thought (giving billions of people around the world "happy moments" in an instant shows power on a vast scale).
in KMC, in comics, in any sort of rational debate, I see nothing at all but a pico second slaughter of Flash by Surfer.
I appreciate that it's fun to play devil's advocate, but some here actually believe that these arguments are legitimate! 😖
ok, just as a mental exercise, check these:
surfer wins via speedblitz and IMPs
surfer wins via speed stealing (PC drainage)
surfer wins via molecular vibration (exciting Flash's molecules to instability)
are any of these outside of the scope of surfer's powerset?
or outside of the scale here?
not at all.
so at the very least you have to accept that Flash wouldn't easily be able to accomplish the very same attacks on Surfer, if Surfer has the same abilities?
well then, what is left? durability and versatility (where Surfer owns Flash), Scope and Scale of powers (ditto), skill and accomplishment with powers (ditto - Surfer's been using the PC for thousands of years, in all kinds of environments, and in many exotic ways).