One of things people learn is that adding indexes isn't free. All write operations (insert, update, delete) will be slower – well, they have to update index.
But realistically – how much slower?
Full tests should involve lots of operations, on realistic data, but I just wanted to see some basic info. So I figured I'll just look at speed of inserting data (well, COPYing data), and will try to extract some knowledge from it…