That shouldn't really be the case. I read you shouldn't connect USB 1 and 2 devices to the hub at the same time otherwise there are no issues with mixing.
But the host controller should assign a mode to your devices to decide how it will allocate the bandwidth, and a hub wouldn't change this.
Mouse and keyboard aren't slow, they just send sort of sparse data, so it it will get an 'interupt' mode, a web cams aren't fast but would get an 'isochronous' mode because it's a streaming device and the external HDD would get a 'bulk' data mode.
I've had no issues mixing these device types on it.
EDIT: lol I can admit when i'm wrong, the theory was right, but taking the mouse off my hub has made a difference.