Can you go to speedtest.net and post back your download/upload speeds on that laptop?
You're probably right, but OneDrive seems to me to be slow whether it's on a Mac or a Windows machine compared to dropbox or others (GoogleDrive), especially on large files. Lots of my Clients think they have a fast ISP connection, and when running on a dual environment, their Mac is at 6MBPS, and their Windows laptop is at 0.6 MPBS. Sometimes other software or viruses get loose on a Windows machine (more common than Mac as you know), and can drastically slow down or remove Internet access entirely (these are variants of DOS--Denial-Of-Service attacks). You should scan for viruses/malware on that Win laptop...have you done this? If...