My oldest Linux host has trouble with IPv6. Apparently it's the only
host in my home network that does have trouble with it (of those that
can do IPv6), and I've run out of ideas. All tests are in the same
network - traffic does not cross into my ISP's area. The same symptom
has persisted across two different switch/router combos - first an old
Airport Extreme, now a TP-Link Deco XE75.

Seeking new perspective on this symptom.

What happens: IPv6 TCP sockets start out fast, then after a few seconds,
they stall for a bit, then recover, then stall again, and this repeats.
This never happens with IPv4, so I usually disable IPv6 for the onboard
network interface with

net.ipv6.conf.eth0.disable_ipv6 = 1

in sysctl.conf.

My other two Linux hosts, one desktop (Intel 4th-gen i7) and one
VirtualBox VM, don't show this symptom.

So far the kernel version doesn't seem to matter.

The troublesome host has an AMD Phenom II X6 CPU from ~2011, uses the
r8169 driver for its NIC, and currently runs kernel package
linux-image-amd64=6.1.55-1. It cannot yet run a later kernel because of
the old nVidia discrete GPU in it - nvidia-legacy-390xx driver won't
compile on later kernels from Devuan repositories.

Today I tried an Apple USB Ethernet adapter (driver ax88796b), to see if
the r8169 driver might be at fault. The symptom followed the network
interface.

Here's lspci. Non-essential devices are disabled in BIOS (embedded GPU,
embedded audio) to make graphics and audio setup for MythTV simpler:

00:00.0 Host bridge: Advanced Micro Devices, Inc. [AMD] RS780 Host Bridge
00:02.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780 PCI to PCI
bridge (ext gfx port 0)
00:09.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780/RS880 PCI
to PCI bridge (PCIE port 4)
00:0a.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780/RS880 PCI
to PCI bridge (PCIE port 5)
00:11.0 SATA controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 SATA Controller [IDE mode]
00:12.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:12.1 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0 USB
OHCI1 Controller
00:12.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:13.0 USB controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 USB OHCI0 Controller
00:13.1 USB controller: Advanced Micro Devices, Inc. [AMD/ATI] SB7x0 USB
OHCI1 Controller
00:13.2 USB controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 USB EHCI Controller
00:14.0 SMBus: Advanced Micro Devices, Inc. [AMD/ATI] SBx00 SMBus
Controller (rev 3c)
00:14.1 IDE interface: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 IDE Controller
00:14.3 ISA bridge: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 LPC host controller
00:14.4 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] SBx00 PCI to
PCI Bridge
00:14.5 USB controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 USB OHCI2 Controller
00:18.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 10h
Processor HyperTransport Configuration
00:18.1 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 10h
Processor Address Map
00:18.2 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 10h
Processor DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 10h
Processor Miscellaneous Control
00:18.4 Host bridge: Advanced Micro Devices, Inc. [AMD] Family 10h
Processor Link Control
01:00.0 VGA compatible controller: NVIDIA Corporation GF119 [GeForce GT
610] (rev a1)
01:00.1 Audio device: NVIDIA Corporation GF119 HDMI Audio Controller
(rev a1)
02:00.0 USB controller: Fresco Logic FL1100 USB 3.0 Host Controller (rev 10)
03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd.
RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 03)
04:05.0 Multimedia video controller: Conexant Systems, Inc.
CX23880/1/2/3 PCI Video and Audio Decoder (rev 05)
04:05.1 Multimedia controller: Conexant Systems, Inc. CX23880/1/2/3 PCI
Video and Audio Decoder [Audio Port] (rev 05)
04:05.2 Multimedia controller: Conexant Systems, Inc. CX23880/1/2/3 PCI
Video and Audio Decoder [MPEG Port] (rev 05)
04:06.0 Multimedia video controller: Conexant Systems, Inc.
CX23880/1/2/3 PCI Video and Audio Decoder (rev 05)
04:06.1 Multimedia controller: Conexant Systems, Inc. CX23880/1/2/3 PCI
Video and Audio Decoder [Audio Port] (rev 05)
04:06.2 Multimedia controller: Conexant Systems, Inc. CX23880/1/2/3 PCI
Video and Audio Decoder [MPEG Port] (rev 05)

-Michael