VrayRT was not a planned feature and was tacked on when it first started development. The main goal of VrayRT was clearly defined as GPU acceleration, CPU functionality was meant primarily as debug and a legacy function. The main problem was the two different functionalities. Vray used exclusively render buckets, which was not compatible with an interactive progressive refinement idea.
So they started deving it, which ment for the longest of times, Vray RT couldn't do half of Vrays materials and features. Now VrayRT is a mature interactive GPU preview system.
Corona did stuff differently. It was planned from the get go to have an interactive functionality and progressive render nature only helped cement that. Basically, Corona IR is almost 100% the same as normal render, the only difference being, that AAvsGI is something super low and thus sampled faster, but doesn't cast rays as efficiently and auto updates. That's it, no real difference.
So this is basically the reason, no code was changed for CoronaIR, while Vray had to reinvent itself. (Which they did a very good job on the GPU side, for such a long running product)
Production Render times are almost equal to me between the two since i still use IM+LC on vray.really noticeable though.
Corona gains much speed by being built from the ground up on Embree, but loses a little bit, by always having Reflective caustics on, while reflective caustics are Default off in Vray and have to be enabled. So a comparison is not really possible. Only thing that really matter in the end is preference.
