What creates the the delay is most likely the fact that the attached document is a PDF, and creating a PDF takes longer than, say an RTF.

Here is how you can change that:
1) Go into "Corporate/Business Settings >> Email Templates".
2) Double click the template you use.
3) Find the tag \ATTACHPDF\ at the end and replace it with \ATTACHRTF\

Then, when you open the "send mail" dialog, it will generate an RTF file, which should be considerably faster.