Is there a slowing of app download speed when an iPad is locked?
"Background" is a confusing term, I don't think Apple refer to it at all, but prefer to use "Multitasking" and "Fast App Switching".  As introduced in iOS4, there are a limited number of functions that can maintain running when it is not the foremost app:

audio / VoIP / GPS &amp; location / push notifications / local notifications / task completion / fast app switching

When you download stuff, say a podcast etc, you are really using the "task completion" service, where the program is allowed to continue running while a download completes, or even an upload etc.  This is limited to (I think) 10 minutes, and really is a request for more time.  Some apps game the system, particularly twitter style apps, by staying running and requesting 10 minutes of task completion time even through they don't have a specific task in hand, and just want to still be there when you flip back (hopefully in less than 10 minutes)...

As such, and of these 7 services are afford the same system priority as any other foreground task, so the only thing that should impact on a "background" download, is either a) the 10 minute slot running out, or b) the foreground app sharing the connection to do it's own thing.

As when you flip back, it will (if it supports the Fast App Switching feature) continue to download anyway, even if the 10 minutes have passed, it may well still have got 10 minutes of extra download, but with more to go.  This can make it look as though it has been downloading slowly in the background, when in reality it's been going full speed for 10 minutes, and then stopped until you switched back.