Having performance issues with encryption

[joelanman]

Hi, thanks for your work on this!

We recently tried it on the GOV.UK Prototype Kit: alphagov/govuk-prototype-kit#1658

However we're seeing a performance hit of about 700ms on each request using the default encryption options (alphagov/govuk-prototype-kit#1843). Is this normal, is there any configuration we could try to improve performance?

Thanks