I must admit, I'm not a heavy user. Uploads with large files are slow, but I think that's due to the asymmetric nature of my cable connection (500Mbps down/50 Mbps up). Downloads to iPhone with large files can sometimes be slow. Otherwise, I haven't noticed any issues. When I say large files and slow, I mean a 300MB file can take about a minute to download to iPhone. I'm not sure if Dropbox would be faster as it's been a long time since I used it. I do have it set up so frequently used files are available offline.