Are you using SQL 2008 R2 or later? If so, are you using the WITH COMPRESSION option? That will give you a pretty good balance between CPU cycles and disk space. When that came out my experience was that the backups were quite a lot smaller and ran much faster. That might be enough. Compressing the file again (and decompressing it again) will take time. The options with the highest compression will make it take the longest because there's more work to do. You can't escape that. It's just a question of finding the right combination for your situation.
Are you opening the backup file with 7zip or with windows explorer? I found that a combination of large file (100GB zipped down to 15GB or so) and encrypted ZIP was a Bad Thing when trying to view / manipulate the file using Windows's inbuilt zip tools. Use 7zip to open the file - much quicker.