Hello Stefan,
I've a similar setup to yours and have similar problems. If you've installed sync without messing the configuration, you did everything right. On my NAS Resilio is also much slower than my laptop or computer. I can't give you a clear answer, but maybe it helps. Here are some of my values I recorded.
My NAS max transfer speed is 100MB/s (Local Network)
NAS ------ sync -----> Laptop (37MB/s)
Laptop ------ sync ------> NAS (11MB/s)
You can reconfigure resilio as Gane recommands, but in my opinion it doesn't matter, nothing changed... BUT when I looked at my NAS while transferring data, my NAS says that I'm transferring the data with 42MB/s not 11MB/s as Resilio said. Even if I look on both resilio windows while I'm transferring data I got two different speed values. I don't know If Resilio calculates the transfer speed correctly. I trust my NAS I tested it with copying data without Resilio and checked the speed and it matches perfect.
How much memory do your NAS have? My NAS has 512MB of RAM, so its totally clear for me that my NAS is constant slower. I can see that Resilio eats all the memory while syncing.
I also own a VPS as always online host to avoid relay server (I recommend that for you too). My VPS is 4xCores 16GB of RAM AND no performance issue, the VPS behaves as my laptop or computer.