It turns out that it wasn't committing enough. In fact I didn;t know when I was committing at all. I discovered that the "sensible" defaults can sometimes not be so sensible. In order to explicitly set a commit count you need to add the following to your import command (highlighted in red):
db2 "import from [your_csv_file_name] of del commitcount 1000 insert into [table_name]([column A], [column B], [column C], ...)"