Speaking of Silver Surfer, I've been reading into the Power Cosmic in between my regular profile updates, and though not halfway done yet, I've noticed two major things (I will say, I probably won't contribute to any of the herald speed/ap tier revisions even though my findings could be super relevant to it).
1. Silver Surfer has several to a dozen feats for time traveling through raw speed. However, there are at least 4-5 statements confirming that he achieves those feats by going light speed or somewhere moderately above it. I'm not sure whether or not that would adhere to our Immeasurable standards, so I'll have to get input on that when the time comes.
2. Nearly all websites don't really define the power cosmic correctly and specifically beyond something vague like "a reality-warping source of power distributed by Galactus." To put it simply, the power cosmic is just the essence of the Universe, and drawing power from the Universe. That's why most of it's abilities are science/physics based. Furthermore, there are consistent statements indicating that power cosmic users draw power from the infinite parts of the Universe, or they flat out scale to the infinite expanses of the Universe. That combined with "infinite power" statements (not something too vague like infinite stamina, Silver Surfer's power level is stated to be "near-infinite" at one point), and there's a solid case to be made for High 3-A, Infinite Speed being a consistent tier for mid-heralds (those who scale to heralds of Galactus).