VP9 compresses video well but is notoriously slow to encode (as are other codecs of its or next generation).
Looking into the HW accel story, don't know whether suitable hardware is (yet) available or budget-friendly, but some positive noises coming from my Google contacts. :)
Intel's new Kaby Lake series of CPUs includes hardware VP9 encode acceleration, though I don't know what tools are required to make use of it. Still worth looking into, but ops wouldn't likely be happy about needed special hardware for special needs.
I'm going to close this one out as getting special hardware for transcodes is unlikely to happen. Stick to software encoding for now which is easy to scale out.