Works as intended, if you cant handle incoming data fast enough, or have write behind cache enable then it will do that.

Itøs your pc that tell steam server to slow down then you cant keep up, its a network protocoll standard, you will not or should be allowed to change this, you can flood your own connection with it, normaæ i will not reply like that, but it is what it is and do as it does. it try prevent resend of all data then you are slow or can't keep up.

Fix or change buffer
try change disk driver's propreties in windows device manager its option 2 if enable try disable it, but sdame time you also turn off buffing up more data then you can handle.

ps.
Many do some rookie mistakes , have defrag running or backup solution laptop pc has alot of this running, or play games same time.

Yes you can do all this is that disk drives do not get use by system, ( also know as double access or 2 things try use drives, that mean performance drop.

And no we have no clue here in how many drives you have or apps running or know what bottlenecks is, also why mnay have RAID drives in the past, seem like SSD is not fast enough anymore. ( still slowest piece of crap in our PC. go ahead google why RAID strpied was a thing in the past and yet STILL IS if you want faster system,

And just wait to you see other user say SSD is good enough, there is no way you can beat 2 drives works as one, that will always be faster. ( This is the diffrence in causul player and other players that want more power, its funny how nothing has change over 40 years with PC .
Raid will beat any single drives then 2 drives works as one.

I point at other ways if you can afford it or want to spend money on it, and how know if your PC even can have it. ( its not my fault or others that PC user today don't know past ways to incresed speed on the system.

gl with it.