With the Digi WR44v2, we run L2TP with PPP autentication over 4G, into our privale MPLS (not with IPSec), and we see a significant drop in spped compared to a direct connection to the provider network (EE). Typical 44Mbs download through L2TP, compared to 80Mbs download to EE. We also see a drop in speed using GRE.
Tests are to www.speedtest.net using WR44v2 (WR44-L100-NE1-SW) FW ver. 18.104.22.168
Extensive testing shows no problems (congestion or otherwise) through our MPLS, we are querying whether additional processor overheads required by the WR44 to route through tunnels affects the throughput.