I've worked with this ... MUMPS is a dead language. Cache is a proprietary implementation of mumps that costs big bucks; cache is interpreted and pretty slow compared to any other language since 1989.
I think the most relevant bit is that it's much cheaper than Oracle. And I've seen Caché do quite complex things quite quickly, like with all database systems it's up to your indices more than anything else.
The entire VA (VistA) and the DOD (AHLTA, CHCS) systems are still implemented in Mumps and will be for the foreseeable future, far from a dead language