Pdf Rendering and merging taking a lot of time

[Sahil333]

I am looking for a better solution to merge a set of htmls into a single pdf. Each html can have any number of pages when rendered to pdf. When the final pdf is printed on a printer with two-page side printing option, each html should be printed as a single unit i.e. no printed page should have content from two htmls. Currently, I am rendering each html to pdf, using pdfjs to get pageCount, then merging each html one by one with a single blank page in between, if the last html had an odd number of pages.
I am using jsreport.render for rendering, jsreport-pdf-utils/lib/utils/addPages for merging, and pdfjs.ExternalDocument for pageCount.
Set of html to pdf conversion is done with Promise.all, so rendering is not a bottleneck when using http-server strategy but the merging is what takes most of time (I'll have to check for exact numbers).
Can you tell me a better approach for this problem? Can I do this in a single operation of rendering with injecting blank page where necessary?

[Sahil333]

Is there any way to know the page count of pdf to be rendered from html before rendering? All the data is already embedded in html