Validating each postcode on a CSV upload takes multiple seconds

[mbarton]

For each row in the CSV upload we validate the postcode against https://findthatpostcode.uk. This takes ~0.8 seconds per postcode meaning the time to process the CSV is roughly the number of rows in seconds. This feels quite slow to the user.

We look up the index of multiple deprivation for each postcode via our rcpch-census-platform API. This again validates the postcode but using postcodes.io instead.

postcodes.io allows bulk validation of up to 100 postcodes. We call it via our rcpch-census-platform API though so we'd need to add a bulk endpoint to that too.

We switched to findmypostcode in E12 as postcodes.io was missing some postcodes. (rcpch/rcpch-audit-engine#558, rcpch/rcpch-audit-engine#576). At the moment if the postcodes.io lookup fails (via the IMD call) then you'll just end up with no IMD. I'm not sure of the consequences of switching everything back. I don't believe findthatpostcode offer a bulk endpoint.

[mbarton]

We'll win 0.1 seconds with this simple fix: rcpch/rcpch-census-platform#23