By definition, there's no such thing, given that mentoring is a relationship meant to examine long-term goals. Speed mentoring events are really networking opportunities in a style often compared to speed dating. It's a way to meet a bunch of potential mentors very quickly.
I find it funny that, yet again, we're disguising "networking" as the more palatable "mentoring". (cheese sauce on the brussel sprouts!) They are two sides of the same coin. We need to build a community that contains (or leads us to) potential mentors. Networking is how we build community; we network every day.
The "speed" events can help you network quickly with lots of folks since research says it takes about 3 seconds to establish if there's a "connection"... (really? hmmmm...) Networking events are a great tool to use but, like speed dating events, you only get to pick from what turns up. Building a community, finding a mentoring, all takes time.