Size could matter, but it should be OK. Is it one long clip or a bunch of shorter clips that add up to 20 GB?
One more thing to try...
Go to this Event folder in the Finder.
Taking care not to delete your original clips, drag the iMovie Thumbnails folder, iMovie Cache Folder, and iMovie Stabilization folder (if applicable) to the trash.
Then restart iMovie and let it regenerate the thumbnails. It may take some time. Let it finish.
That may help if the thumbnail creation was interrupted when you first imported.
There are about 400 clips wich add up to 20gb. I also tried your last suggestion. I found that sollution on the internet. Also did not work.
The only thing I can do is finish my movie without previewing in the Project window. If I don't do that than the skimming will work.
I have edited severall movies now and encountered this problem before but than the audio skimming would work. Restarting Imovie always solves the problem but I think it might be a bug or something.
I have contacted Apple. I had three days of free support left. They helped me by restarting my Mac by pressing the keys: P+R+CMD+SHIFT en than the power button. (I don't know what that is for). After that my Mac played the start up sound twice. Then I did a "Repair Disk Permissions" excersise in the Disk Utility programm.
After that my Mac was much faster in starting up. The problem was probably that I rarely shut down OSX. Usually I just put my Mac to sleep. They advised me to just Shut down my Mac every night.
One more thing.... I am using Imovie a lot. I have read that when the programm is working slower I can empty the cache folder. I also read that this can harm the programm and make it even work slower.
What is the wisest thing to do?
As for me, I never shut my Mac off, because there are things that it does at night to keep itself healthy, and I have automatic backup routines that run at night. I just shut down the display and the disk drive and let it run.
About once every week or two, I will shut it down and restart, to clear out the memory.
I have an iMac. If you have a MacBook, people may do different things. There are discussions for every type of Mac. I suggest you ask the regulars for the type of Mac you have what they do.