[Tinwarble]

My external drive is a brand-new Passport USB-3 drive (4TB), connected to the Shield with the 2 or 3-foot long USB 3 cable that came with the drive. I'm copying approximately 1.8TB from one source (the Windows 7 HTPC) and another TB from a NAS.

Before I go much further, I want to ask a question that Tinwarble addressed earlier: sending files to the Shield from the Windows 7 PC. For the sake of keeping everything streaming as fast as possible, wouldn't it make more sense to have the source directly connected to the playback device via a USB-3 cable at 5Gbps than over a LAN with two gig-E switches at 28 MB/s?

I'm assuming that you are getting the 28MB/s from copying files to the Shield, that is the write speed which is always going to be slower than the read speed. The Shield will be able to read the streamed files much faster (usually double or more), depending on your source. So don't use your write speed to determine what you will be able to stream.

And consider this, the majority of people use a NAS, whether it be a true NAS or just something they use as network storage, to stream files to their Kodi devices. I personally, at any one time, stream multiple videos from a single NAS to multiple Shields.

So no, you are not going see much difference streaming over your network vs playing files from an attached drive. As I said, the max bitrate for a blu-ray is 48Mb/s, that's 6MB/s, and even if you take your 28MB/s that is still more than enough bandwidth to stream without issues (as long as you don't have some issue with your network).