Slow rendering is usually due to a complex effect, though it could also be due to a full hard drive. There's no setting for a scratch disk, it'll just use the library's location for render files, if any. RAM probably isn't relevant, but you have more than 1GB!
Essentially, make sure your library is on a fast drive with plenty of space and most effects will render quickly. 80GB is not a lot of free space on most drives, though.