The simple answer is to just preview the project, but I'm guessing that the combination of media clip encoding/resolution and your PC's computing power won't let you preview smoothly. Speeding up a clip takes a lot of computing power and previewing sped-up HD or 4K clips will be too much for many systems.

Your 2 main choices are to lower the preview resolution (which will reduce the video quality but improve playback performance), or use range select to define a specific section of the clips and then use Range Produce to just produce that one section.

There is also a Render Preview tool when you have a range selected, but in my experience it takes a very long time and isn't very useful. Producing the entire video is usually way faster than rendering the preview of just one section.

You can also try using shadow files, which create lower resolution copies of your source clips for PD to preview, but depending on the specific speed increase you're using, PD still may not be able to preview smoothly all the way through.

Thanks for replying so quickly. Alas as you said it's too much for my computer to preview the whole thing! Several hours later - the video speed was great and I got the audio working alongside it, but I decided it was too long to hold people's attention so I have done a lot of trimming. My problem now is that I can't figure out how to put the audio alongside the video. Do I insert the audio track just once, or keep inserting it right along the video even though the video is now more or less the length of the audio? Preview doesn't show the video sped-up.

Thank you for your help.