Is the DS218+ perhaps connected to a different place in your network than whatever you were using before?
Have you tried going to http://atv.ip.add.ress:57000/speedtest with a browser (the Channels app has to be up on the ATV, but doesn't have to be doing anything) to check the performance between that ATV and your DVR? When I do that I get numbers like 995Mb/s download, 1-3 ms latency, 0.11-0.14 ms jitter. That's going through two NetGear ProSafe GigE switches.
Maybe try them more extensively so as to narrow-down the problem?
We have seen people encounter such things on wired networks, before, where the problem was the LAN switch.
Here's another thing you might look at on your Synology NAS:
[email protected]:~$ ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:11:32:8E:76:9B
inet addr:172.16.1.2 Bcast:172.16.15.255 Mask:255.255.240.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:19640371577 errors:0 dropped:0 overruns:0 frame:0
TX packets:6161615740 errors:0 dropped:0 overruns:0 carrier:0
RX bytes:24005278017398 (21.8 TiB) TX bytes:4264363648768 (3.8 TiB)
[email protected]:~$ uptime
14:30:00 up 86 days, 3:20, 1 user, load average: 0.00, 0.05, 0.09
You'll notice there are zero errors, dropped packets, overruns, etc. in 86 days of uptime, with 22TiB of data received and 4TiB sent.