Time for a simple one.
Let's just assume, for a second, that we don't much care for Multiversal UT or whatever, and that Assy's only affecting one universe with his "purging of a timeline". I sort of doubt that would be something he'd joke about/exaggerate, it's pretty obvious people who can save/load can affect the world on a universal scale already, and this is after he stops expressedly holding back (during which time he's throwing around stars at you apparently?)
Anyhow, here's the fight in question if you haven't seen it a kajillion times.
The timestamp for the first shot is 589.41 seconds, for the second shot it is 600.09 seconds. This means he pulled the whole of the universe into his glowing orb of light/devouring goat skull/whatever in 10.68 seconds, so there's our timeframe. All that's left to do, since this is drawing in the universe from all directions, is to measure the radius of the observable universe:
~8.8e+26/2 = ~4.4e+26 meters
4.4e+26 meters/10.68 seconds = 4.11985e+25 meters/second, or 137423403760210670 C.
For those not skilled in the translation of "biggaton" numbers, that's a whopping ~137.423 quadrillion times the speed of light. "Undertale caps at Massively Hypersonic" my a%$.