Learn Spanish (Latin America): Esta horda implacable es hiperrápida.

Because the prefix hiper- ends in an 'r', and the base word rápida begins with an 'r'. When you attach them, the two 'r's sit side by side and form the rr digraph: hiper- + rápida = hiperrápida. This is different from prefixes that end in a vowel, where you have to artificially double the 'r' to keep the rolled sound (like anti- + robo = antirrobo).

Is hiper- used interchangeably with super- and re-?

They are very similar, but they carry slightly different levels of intensity. Super- and re- are incredibly common for everyday emphasis (like "really fast" or "super fast"). Hiper- takes it a step further to mean "excessively" or "extremely," adding a slightly more dramatic, exaggerated tone to the sentence.

Why use es instead of está for being fast? Isn't speed a temporary condition?

While a person or thing might be temporarily moving fast right now (which could use estar), here the sentence describes the fundamental nature of this specific horda (horde). Because being hiperrápida is an inherent, defining characteristic of this group, you must use ser.

Could I write hiper rápida as two separate words?