I'm uploading summary and detail records. There are around 70,000 detail records, and about 10,000 summary records.
The 70K loads in about 1 minute, while the 10K takes about 8 minutes.
Why does it take 8 times longer to load 1/7 the data?
There's a number of factors that could come in to play, including triggers, workflow rules, processes, rollup summary calculations, sharing calculations, current system load, database contention/row locks, etc. Realistically, to find out the exact reason, you need to contact Support, as they can look at the detailed logs to find out why your performance may be slow. Try inserting a summary and detail record manually and check the logs to see how long they take to insert. This may give you an idea of why there's a difference in timing.