Optimize the genpdf function

[plumsirawit]

It takes extremely long (i.e. 30 - 60 secs) to generate a pdf. If we could improve it this would be good (yes, for practical uses, of course). However I doubt we'd have to change the underlying stack of python, wkhtmltopdf, cpdf.

[plumsirawit]

Currently the "short" polling interval is 5 secs (for economical and performance reasons, balanced).