Not intrinsically slow, probably slow when run on 20GB of unindexed data..
something x2...x3 times slower than just copying 20gb of data
I would consider "slow" a database query that takes that much.

That's why a nice indexed structure is a huge advantage and several tools certainly exist to build an efficient blockchain database and execute fast queries on top of it. Unless it's about practicing, as Buffer Overflow pointed out.