As someone who just indexed 6m documents with pgvector, I can say it’s a massive time sync - on the order of days, even with a 32 core 64Gb RDS instance.
Though index creation is not a big deal, I want good queries rapidly for cheap. So IMO RDS with pgvector is the easiest approach.
Your comment makes it sound like Marqo is a way to speed up pgvector indexing, but to be clear, Marqo is just another Vector Database and is unrelated to pgvector.