Which version of Meilisearch were you using?
We chose to use LMDB instead of RocksDB because it is a much faster, memory and CPU-efficient key-value store. However, it would probably be much quicker to insert all those inverted indexes into RocksDB. RocksDB is an LSM tree, meaning that merges must be done to retrieve the different entries, which means that the search side of the engine would suffer from that. LMDB is much easier to operate. There are a lot fewer parameters to manage the user-end cache, etc.
The bottleneck of the Meilisearch indexing phase is not writing into LMDB. It is mainly related to computing a bunch of databases storing prefix words and the computation of other databases. We are working on making that much faster and reducing the index size on the disk.