as an experiment I closed down all browsers and reboot and just let it sit idling.
Now it fared much better, memory went from 96m free to 87mb after 23h.

root@GL-AR750S:~# free -h
total used free shared buffers cached
Mem: 124608 63380 61228 384 8632 17652
-/+ buffers/cache: 37096 87512
Swap: 0 0 0
root@GL-AR750S:~# uptime
06:42:53 up 23:26, load average: 0.29, 0.25, 0.25