I've been having the same problem and the other thread mentioned above works, although it is also slow.
But I tried it using Automator and built a workflow that can modify a large number of files in seconds. As you said you are new to the Mac world, I'll assume you don't know how to do that.
1) Open Automator from your Applications folder.
2) Select iPhoto from the left menu
3) Choose Find iPhoto items, drag it to the right and then enter the criteria you want it to look for (works best for me if I do an album at a time)
4) Then Select Preview from the left menu and choose Open Image in Preview and drag it to the right
5) Then choose Scale Image (It will prompt you whether you want to make copies of the photos which you will be using; it's always a good idea to have backups), drag it to the right and scale to the size you want (I find 680 to be plenty)
6) Select Finder from the Left menu, choose New Folder and drag it to the right. Now you can specify the folder name and location (Desktop is the default)
7) Hit the Run button in the top right corner.
8) Your photos should now be able to upload several photos onto Myspace without any problems.
9) Save the workflow you've created for future Myspace photos
As for general photo speed, I have found that downloading Safari 3 Beta has made it faster. Download it here:
http://www.apple.com/safari/download/
Hope this helps!
