Just installed Photo Station on my DS214Play and it's just indexing all my photos - of which I have quite a few - so we'll see what it's like once that's done. I'm using an iPhone 6s as my remote device.
Initially it doesn't seem unreasonably slow but my DS is very much quicker than yours anyway. As an example I can hit about 90MBs on file transfer so getting close to gigabit speeds, it can also do on-the-fly video conversion.
I'll be back once all the photos have been indexed.
Having installed DS Photo/Photo Station and played around with it a bit I find that it works as one would expect.
I don't use any BT devices having an Asus modem/router. Obviously connection from my iPhone is via wifi and my NAS & PC are hardwired , via TP-Link switches , to the Asus.
If I use my phone to upload/download photos then that happens almost instantly. If I use Photo Station on the DS to download to my PC they are zipped up and transferred again virtually instantly. The test I did was 162 photos totally 195MB.
Not sure if that helps at all but I'd suspect that as all traffic would seem to be going via the Orbi (I think based on how you've described your setup) then that might be the area to look at.