There’s a number of factors which can affect performance in DominoPDF.
Firstly, it uses HTTP / HTTPS so if traffic is busy or you have a slow connection etc. speeds can be affected.
Next, DominoPDF processes HTML from a URL which can come in may different forms. Some URL’s will return plain HTML, some will return HTML with CSS, some will return HTML with embedded styles, external references, images etc.
DominoPDF will need to retrieve all the required elements (over HTTP) and then render them internally (HTML, CSS, images etc.) before the PDF conversion takes place.
So you can see that depending on the source, the time it takes to generate the PDF can take differing lengths of time.
Best performance will be seen with a good HTTP connection and “clean” HTML returned by Domino or the URL source.