That could just mean it’s doing more hashing faster, hence putting a heavier load on the disks. If this is a Linux box it’ll by default use all CPUs for hashing. Set the “hashers” option to something lower to reduce load.
You should probably first determine whether disk IO or CPU is the bottleneck (e.g. using top and iotop).
Personally, I must admit I like systemd to manage system services. It allows me to drop a file like
[Service]
Nice=10
CPUQuota=60%
into /etc/systemd/system/syncthing@.service.d/resources.conf and I never notice whether Syncthing is doing some heavy lifting or not. However I must admit I do not know if restricting to 60% of cpu time (I think that’s what CPUQuota does) or e.g. to 3/4 cores (by reducing hashers) is more effective.
Sorry for not getting back. I was very busy during the last days and observed in the meanthime carefully what syncthing was doing. I think the major issue is not performance in terms of PC capacity, but the uploading of data to the other machine. That slowed things down considerably and made some apps which use the net unusable (like browser…). I edited the maxupload to a small amount (20 kiB/s), which worked fine then. But I fear this would take ages! But I have a new problem. It seems that the PC doesn’t connect to the NAS any longer. I’ll check here on the forum about it and maybe post there if I don’t find a solution.