I deal with large CSV at work, mostly database dumps. Google Sheets and Numbers/Excel just can't keep up with formula changes for 300k+ lines. What do we do? We use Python.
The use case here is replicating a the VLOOKUP function with a left join. We want to get the matching criteria from our referenced CSV file, but only the matching (otherwise that would be a full outer join).
Here's what our data looks like: