What's being measured: each library runs the same query, SELECT id, title FROM topics ORDER BY id LIMIT 1000, and walks all 1,000 rows, building one string out of every id and title. That's one iteration. A higher iterations/sec number means less Ruby-side overhead for turning PostgreSQL rows into Ruby objects.
Because the database work is the same for every library, the raw pg gem is the floor. Anything above it is the cost of the library's abstraction.
Script: bench/topic_perf.rb