O problema básico: Ao atualizar um jogo através do Steam, eu recebo um pico inicial de atividade de rede, então o download pára.
Conforme exibido pelo Steam, a velocidade diminui:
Aoanalisarotráfegoderedereal,eleaumentaumavezedepoismorretotalmente: 
Algumasnotasaqui:
- Quandoodownloadéreduzidoa0,eleficaem0.Pausaresoltarodownloadnãotemefeitonão.Parareiniciarodownload,precisoreiniciaroSteam.
- Osdadosreaisaparentementesãorecebidosporessesegundo,portanto,nãoéumcasodebloqueiototal.DapróximavezqueeuiniciaroSteam,ospoucosMBdedadosquebaixeiaindaestãolá.(Emteoria,eusuponhoqueeupoderiaobterodownloadde1GB,reiniciandocercade200vezes...)
Agora,aspartesinteressantes:eupossoterumdownloadmenosinstávelcolocandoumaaceleraçãoseveranalarguradebanda. 
Dessaforma,possoobteralgumascentenasdeMBdedadosemvezdecincoMB.Noentanto,aindaassim,eventualmente,sufoca.
EuiniciooclienteSteamapartirdeumconsole.Asaídaébastantedetalhada,masaquiestáumexemplo:
RunningSteamonubuntu15.0464-bitSTEAM_RUNTIMEhasbeensetbytheuserto:/home/ernir/.local/share/Steam/ubuntu12_32/steam-runtimeExecCommandLine:"/home/ernir/.steam/root/ubuntu12_32/steam steam://open/driverhelperready"
ExecSteamURL: "steam://open/driverhelperready"
CAPIJobRequestUserStats - Server response failed 2
ExecSteamURL: "steam://open/downloads"
Generating new string page texture 127: 128x256, total string texture memory is 5,28 MB
Installing breakpad exception handler for appid(steam)/version(1437790054)
Installing breakpad exception handler for appid(steam)/version(1437790054)
Installing breakpad exception handler for appid(steam)/version(1437790054)
Installing breakpad exception handler for appid(steam)/version(1437790054)
Generating new string page texture 128: 128x256, total string texture memory is 5,41 MB
process 4301: arguments to dbus_connection_ref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2656.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_unref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2794.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_unref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2794.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_ref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2656.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_unref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2794.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_unref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2794.
This is normally a bug in some application using the D-Bus library.
process 4301: arguments to dbus_connection_unref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2794.
This is normally a bug in some application using the D-Bus library.
Generating new string page texture 129: 24x256, total string texture memory is 5,44 MB
Generating new string page texture 130: 256x256, total string texture memory is 5,70 MB
Generating new string page texture 132: 48x256, total string texture memory is 5,75 MB
A linha CAPIJobRequestUserStats - Server response failed 2 é a única que parece promissora, mas procurar por ela não me levou a lugar nenhum.
Informações adicionais:
- O Steam funciona perfeitamente na partição do Windows da mesma máquina.
- Os problemas de DNS que alguns mencionaram não parecem ser o problema - ou pelo menos , instalar e reiniciar o dnsmasq não fez nada.
Estou executando o Kubuntu 15.04 agora, o problema era idêntico no Kubuntu 14.04 e 14.10. Informações do sistema (conforme relatado pelo Steam):
Processor Information:
Vendor: GenuineIntel
CPU Family: 0x6
CPU Model: 0x3a
CPU Stepping: 0x9
CPU Type: 0x0
Speed: 3800 Mhz
4 logical processors
4 physical processors
HyperThreading: Unsupported
FCMOV: Supported
SSE2: Supported
SSE3: Supported
SSSE3: Supported
SSE4a: Unsupported
SSE41: Supported
SSE42: Supported
Network Information:
Network Speed:
Operating System Version:
Ubuntu 15.04 (64 bit)
Kernel Name: Linux
Kernel Version: 3.19.0-25-generic
X Server Vendor: The X.Org Foundation
X Server Release: 11701000
X Window Manager: KWin
Steam Runtime Version: steam-runtime-release_2015-06-12
Video Card:
Driver: NVIDIA Corporation GeForce GTX 650 Ti/PCIe/SSE2
Driver Version: 4.5.0 NVIDIA 346.59
OpenGL Version: 4.5
Desktop Color Depth: 24 bits per pixel
Monitor Refresh Rate: 59 Hz
VendorID: 0x10de
DeviceID: 0x11c6
Number of Monitors: 2
Number of Logical Video Cards: 1
Primary Display Resolution: 1680 x 1050
Desktop Resolution: 3360 x 1050
Primary Display Size: 18,07" x 11,65" (21,50" diag)
45,9cm x 29,6cm (54,6cm diag)
Primary Bus: PCI Express 16x
Primary VRAM: 1024 MB
Supported MSAA Modes: 2x 4x 8x 16x
Sound card:
Audio device: Realtek ALC887-VD
Memory:
RAM: 16001 Mb
Miscellaneous:
UI Language: English
LANG: is_IS.UTF-8
Microphone: Not set
Total Hard Disk Space Available: 104234 Mb
Largest Free Hard Disk Block: 11192 Mb
Todo e qualquer insight é apreciado neste momento!