Did you definitively narrow it down to a network issue yet? If so, and you’re able, plug a computer into each switch and work your way out from there to see where the delay is. It might be as simple as a bad cable someone down the line.
Some other thoughts:
Is WHS up to date?
Any add-ins which might be interfering?
Are you using the drive pool? If so, can you move the file to the C: or D: partition and share the folder, then try streaming it from there?
