>>146,147 Waititi said recently in a Reddit AMA that Banner will steal the show and that he made sure Thor was the most interesting character, so that Loki's part might be modest, giving him time to do some voiceover work, makes sense.
Of course it's also possible that the voiceover work required little time, after all he is voicing a minor character (basically fate repeating itself, Redmayne on the elephant, TH as leg).

Thor3 has 2 more weeks, this is the only announcement so far, we'll see if anything else pops up, in the meantime his stans will do like he does, puff up this small part like it'll get him an Oscar.