>>95929508
>countering speedster powers
You could start by making terrain difficult to traverse, especially at high speed. Things like spreading glue, lube, or ice around, turning the ground into a viscous muck, or better yet — making it a non-newtonian fluid, with speedsters sinking while others can move on it with minimal disturbance.
The greatest asset of speedsters is their mobility, after that often comes their perception. However, forcing them to constantly make use of it to dodge attacks or projectiles, for instance, would cause mental exhaustion from protracted fighting, despite it taking moments for others. This, in turn, would directly impact their performance, weakening or even exhausting them, eventually slowing them down enough to strike by non-speedsters, or catch them with one of the above methods.
>What kinds of things can be done with wavelength changes besides microwaving people, BTW?
For starters, this could drastically weaken the power of light-based attacks against the power user. Turning combat laser into radio wave emitter would keep the energy output, but individual photons would scarcely have enough energy to cause serious damage and would be easier to stop.
On the other end of the spectrum is turning sunlight into high-energy gamma rays, which shred DNA and degrade materials, cause fission, and so on.
You can also wreak havoc on communication networks by fucking with wavelengths of transmissions.
Different wavelength penetrate different materials. For instance, certain wavelength wouldn't pass aqueous humor, thus making them safe for use in combat lasers by the virtue of reduced risk of damaging reflections and backscatter.
Things like radar stealth are also possible through changing the radar's reflected pulses into something that won't be picked up.