Bukkit

 SQL (MySQL, MariaDB, OracleDB, even MS-SQL) stores data on disk, but indexes and optimizes the way the data is accessed in such that very large amounts of data can be sorted, organized, and called upon very quickly.  This is done partially with the data being loaded into RAM (done by the SQL server).  The initial load out from disk to memory will take a moments time, however once this is loaded, all calls the the player's data will be extremely quick.  This should be an option for admins to use, not a requirement.  This could potentially allow you to use SQLite flat-file databases as well, which are still more organized than YAML files, and can more stably support larger datasets than YAML files.

There is not optimizations for flat files without loading the entire file into memory, and even then that's not something done well (or at all) with Minecraft plugins.

Additionally, SQL storage would allow the cross-server inventories without little to no additional code in the plugin.   If done right, the SQL database can hold a single set of shared data, such as player uuid and name and such in one table, and have a separate table for each server.  This would allow optimization of datasets so that there is very limited duplicate data in each table, which would be 100% duplicated in every YAML file on every server.  Specific servers could be set to use the same table within the database to have a "poor-man's" global inventory.  Another great thing with using MySQL is that another table for shared/clan/guild inventories could be created, with the clan/guild name being added to the users table. 

It's very much worth learning MySQL. You can find online tutorials, guides, instructions, free courses, paid courses, books in digital form and physical books too.  I'd suggest getting Mike McGrath's "SQL in Easy Steps"