The default installation of tumbleweed (Leap 15.2 as well) is configured in a
way that *all IPv6 packets* are sent to the router ��� even those packets which
destination is inside the LAN. (Layer-2 destination is always the MAC address
of the router.)

This report is IPv6 specific. IPv4 isn't affected at all.

System setup: 
- Fresh Tumbleweed 20180814 (with KDE)
- NetworkManager (not wicked)

Investigation:
Here are some information of the system:
# ip -6 route
::1 dev lo proto kernel metric 256 pref medium
fd00::/64 via fe80::c225:6ff:fe08:eab8 dev enp0s3 proto ra metric 100 pref
medium
fe80::/64 dev enp0s3 proto kernel metric 100 pref medium

The fe80::c225:��� is the IPv6 address of the router.

# sudo /sbin/sysctl net.ipv6.conf.enp0s3.accept_redirects
net.ipv6.conf.enp0s3.accept_redirects = 0

Explanation:
According to the "ip -6 route" output all IPv6 packets are leaving the computer
from device enp0s3 and are sent to the router (via fe80::c225:���).
I turned wireshark on in order to capture the packets I could see there is a
packet from the router:
It���s an ICMPv6 redirect (Code 137) which tells the sender (the tumbleweed
installation) that he should not send the packets to the router because there
is a better connection available. And tells him the MAC of the destination
which should be used instead of the routers MAC. Thus, for further
communication the packet should go direct the destination and not via the
router. That *would* be OK.
However, tumbleweed ignores the ICMPv6 redirect message because of
"net.ipv6.conf.enp0s3.accept_redirects=0 and keeps sending everything to the
router.

*This ends in a huge performance regression and the router becomes the
bottleneck of the network.* (Huge file transfers will be sent to the router and
the router will forward it to the destination ��� which is in the same subnet!)


Workarounds:
1. Do "sudo sysctl net.ipv6.conf.enp0s3.accept_redirects=1"
The redirect instructions from the router are accepted. Performance will be
great.
This variation is used by the distribution Linux Mint 20 (Cinnamon) and Neon
(KDE & NetworkManager).

2. Add an additional line in the route table (with a better metric than the one
with the ���via fe80::��� line):
# ip -6 route
fd00::/64 dev enp0s3 proto ra metric 100

The packets are not sent to the router in first place and so, the performance
will be great.
This variation is used by the Distribution Ubuntu Server 20.04.

3. Modify the route table entry (=remove the ���via fe80::������ string).
# ip -6 route
fd00::/64 dev enp0s3 proto kernel metric 256 expires 7140sec pref medium

This variation is used by Tumbleweed when you switch from NetworkManager to
wicked in yast (plus reboot).
Performance will be great.


Conclusion:
NetworkManager is creating route table entries with
fd00::/64 via fe80::c225:6ff:fe08:eab8 dev enp0s3 proto ra metric 100 pref
medium
and tumbleweed uses 
sysctl net.ipv6.conf.enp0s3.accept_redirects=0
This is a bad combination. (Fedora 32 had the same combination with the same
bad results.)

Proposal:
Either
# sudo sysctl net.ipv6.conf.enp0s3.accept_redirects=1
or
# sudo ip route add fd00::/64 dev enp0s3 proto ra metric 90 pref medium
(or both) would help to fix the issue.