Purging Performance - Journals flying but Balances slow

I'm running archive / purge on a GL set of Books that is set up with 365 periods per year (initially in test/dev environments).

I'm archiving / purging a year at a time (ie 365 periods) - the journal purges are flying (eg 9.5 million je lines in 6 minutes, 2 workers), but the balances are taking about 2 minutes per period (30000 rows) to purge - at this rate it will take about 48 hours to complete (as it already did in one environment).

Any clues ? I've read what I can find on OSS but most of the performance options available appear to handle the journal purging, not balances.