As shown on the
File Quota section of Help, the maximum file size you can upload is 50 MB. I find that 60 MB files usually work, but 80 MB files fail. Also see the
Bandwidth Limits: files and websites section of the
Account Limits help page.
By dragging multiple files from Windows 10 File Explorer into a folder in the Fastmail Files screen, I get about 2.3 MB/sec file upload rate for a 103 MB total size upload. That's about 18.5 Mbits/sec using my FIOS fiber internet connection with an upload throughput to a test server of about 85 Mbits/sec.
My employer uses box.com for large file transfers, and I find that Microsoft OneDrive also works very well. Fastmail really isn't designed for heavy cloud storage uses.
Bill