Depending on what channels you're watching, they could be different codecs and different bitrates, some of which don't need to be transcoded to be sent remotely, others do.
So without knowing specifically what channels on what devices, it's hard to give you more detail.
The transcoder on the Synology can transcode at about 1.5-2x, if I remember correctly, so you're basically never going to see download rates higher than 2x whatever the bitrate is that you're transcoding to. There's little need for concurrency at that rate because it can likely download the segments as soon as they are available one at a time.
Setting the quality to Original means that you don't need to involve the transcoder at all, so it will be able to download at whatever the rate of your home upload speed and remote download speed are. If you're on Original quality, it'll work great when you have lots of available bandwidth, but will generally not be participating in our Adaptive Bitrate system, which gracefully lowers the bitrate based on current network conditions.