I also asked about this on reddit but have not found a solution. Google finds little help.
I upgraded my home server running Debian Buster to Bullseye and the new kernel causes my Intel ethernet adapter to work really slow with high unsteady latency. Booting back to Kernel 4.19.0-17-amd64 fixes the issue.
My card is a "Kalea Informatique Network Card PCIe 2.0 1x 4 Ports Quad Gigabit Ethernet - Chipset Intel 82576"
lspci:
04:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
04:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
05:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01)
05:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network Connection (rev 01
Ethtool gives this info on the adapter:
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Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: yes
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tx-udp-segmentation: on <- This is off on the working old kernel.
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tx-tunnel-remcsum-segmentation: off [fixed]
tx-gso-list: off [fixed]
rx-gro-list: off
macsec-hw-offload: off [fixed]