Why does it keep going really slow on all those above? More so when I speed each video up too. I can't tell what it is going to look like whilst editing. Sometimes its just guess work. I have deleted all old videos, cache etc. What could the problem be please? I have a mac pro.
Your description is confusing. Project rendering and output rendering are different processes. Saving of projects is different than exporting a finished video. The source of your clips, the project settings and many other things are involved. What does deleting a cache involve on your computer?
With the limited amount of information you've provided, users here will not be of much help. Consider volunteering a lot more information!