Slow PDF generation

[gwiedeman]

Is your feature request related to a problem? Please describe.
PDFs generate relatively slowly, meaning that large set of email with say 10k+ messages can be not very practical.

Describe the solution you'd like
There might be a faster way to do this with Puppeteer. Looks like you can keep a headless chrome open instead of booting a browser each time, which may save significant speed.

Additional context
This isn't a priority for current development, as adding Node to the dependency stack might be a challenging requirement. It's a good idea to make this an optional plugin.