So, I tried replicating this, thanks to @dmai, who painfully filled an external HDD with tons of data.
I’ve a branch for you: https://github.com/dgraph-io/badger/tree/mrjn/try-to-speed-open
This branch tries to make opening tables concurrent. However, in my tests with this external rotational HDD, the process was slower, because random seeks are slow. If you have an SSD, can you try with this branch and report back? @vrischmann
This branch also added logs to show how table opening is going on.