Opening PDFs with signatures is slow
I am afraid I cannot be any more specific than this. I just noticed a file takes considerably longer to open after it has been signed (only once and by me).
I do my observations using Okular, but I think it has to with something that Poppler does. I tried profiling Okular and Poppler while opening the files, but there wasn't anything obvious that jumps out right away. I noticed that as soon as there are signatures in a PDF, there are a couple of 10k calls to SignatureHandler::updateHash (and implicitly NSS's HASH_Update). This seems like a lot and that number does not scale with the number of signatures in the document,
Wild guess: NSS takes a few seconds to initialize and during that time Poppler builds up a huge queue of tasks and retries until NSS is available ... or something in this direction. It would align with the fact that starting the PDF signing workflow in Okular stalls the application for many seconds if I start it right away, as soon as an unsigned PDF is displayed on screen, while there isn't any delay for PDFs with signatures.