According to the guy at Cakewalk customer service, if your time limit expires during a download, you will have to start that download over again.
That's actually incorrect. The 300 minute limit only prevents you from STARTING the download again after 300 minutes. It helps prevent file sharing/piracy. If it takes longer then 300 minutes, it will not stop your download in the middle of the process.
FWIW, the files aren't hosted on our servers, they're served up by Amazon Cloudfront. If you're being limited to ~ 90 kbps it could potentially be your ISP, AV software, or a browser setting. Definitely give Chrome or Firefox a test if you haven't done so already.
EDIT: Also, for clarification, if your links time out just log out of your account and then back in. New links will be generated with new 300 minute windows.