I have submitted price proposals to hundreds of projects and won/completed the majority of them, so I have gotten a pretty good sense of what presentation design activities cost time, and which ones do not. Use them to your advantage when negotiating a project with a freelancer.

Project time wasters:
  1. The most important one: unclear story that needs sorting out
  2. Lots of physical meetings
  3. No clear decision maker on the client side, i.e. freelancer needs to collect and synthesise feedback into one voice
  4. Requirement to run 2 parallel versions of the same presentation (different language, different audience)
  5. Start and stop, pick up the project after a hiatus that was long enough for everyone to forget what it was all about
  6. Inconsistent PowerPoint masters, i.e., client provides feedback pasted in the default PowerPoint template so all colours, fonts, and slide formats are messed up
  7. Requirement for a text 1-pager that summarises all, with lots of iterations on the exact wording of the text
  8. Related: communicating micro edits over the phone to the designer, rather than quickly doing them yourself in PowerPoint (text edits, slide order changes, etc.)
You that slide count does not appear in the list. Adding a few slides with a few new concepts to an existing presentation deck. Once you have understood the story/company, the marginal time required to add things to a presentation is low. Of course a 100 page deck takes more time to complete than a 10 page presentation, but still.