DS-Lite Performance very poor

I bought a Turris Omnia 2 GB and want to run it behind a DOCSIS Modem, my problem is that the performance is very poor on IPv4 (only 20 MBit/s), it will be done via DS-Lite (RFC 6333).
If i let a Fritzbox establish the DS-Lite Tunnel i get the usual 450 MBit/s. But doing Modem -> Fritzbox -> Omnia is not the purpose for buying a 320€ Router.
What i saw so far is that if the Fritzbox does the Tunnel the MTU of the IPv4 Link is 1432, if the Omnia establish the Tunnel the MTU is 1376, i don’t understand why.
I think the performance is a bug because doing a IPIP6 tunnel should even run on a raspberry pi faster and the Omnia is much faster than a raspberry.
Thanks for help

what infos are needed so you can help me?

Constructive criticism is welcome, please help

Hi, what modem do you have and how do you have it set? (bridge? DMZ?) can you check negotiated link speed on port when connected to TO vs Fritzbox?

Modem is a TC4400. It is a bridge and has no router function. Port Speed is in the complete chain 1 GBit/s. I get 450 MBit/s via IPv6 so it should not be a port speed problem.

It seems like on OpenWRT 17.01.4 the problem is not present. source (https://www.unitymediaforum.de/viewtopic.php?f=53&t=36774&start=125) Does the Turris OpenWRT differ from the vanilla OpenWRT in regard to how ipip6 is done on kernel base?