By default Windows 11 encrypts drives using bitlocker.
This requires processing power and slows your drive's processing speed down by more than a half, unless of course your SSD has hardware encryption support and your motherboard has a dedicated TPM chip installed.
Most people do not and use either fTPM (amd processor) or Intel Platform Trust Technology (PTT), rather than an actual TPM 2.0 processor.

Even when you bypass the TPM check during windows 11's install, people tend to forget to disable bitlocker encryption. So after the OOBE screen, their disk/ssd starts being encrypted. (and yes, currently, this is the only reason why Windows 11 even requires TPM 2.0, although it can also encrypt using the older 1.2 version oddly enough.)

This is one factor.

The second factor is that Windows indexes a file and updates it constantly so long it is writing this file, which also slows down writing (because it needs to check it)

A third factor is because Windows Defender actively scans the file per update to check it for virusses (by default at least). The AI tech makes this process even slower.

A fourth factor is windows update, which prioritizes over and above everything, much like a number of other srvhost + rundll stuff windows runs without you knowing or having control over. Make sure nothing runs on the background.

Windows isn't exactly optimized for downloading at your maximum speed. In fact it by default makes sure you cannot use that last 10% of your internet speed so that it can communicate telemetry to microsoft and windows updates to your machine. This requires registry editting to disable.

Steam's downloading runs as a background service on your machine, because of tricks using the Steam Client Service. This means that the download slows down when the computer detects User activity. The user's activity, much like everything else, is prioritized over Steam's download, which Windows thinks is just a background thing.
-- This is why a regular download through the browser is often faster (it is seen by windows as a User activity).

The list is longer.

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Just note the following.
Steam's connection is encrypted.
Steam's files require verification upon download, which means some uploading.
To verify the file, the file needs to be decrypted from your harddrive/bitlocker's encryption, then run through a calculation, then the result is encrypted by the client and send and checked on the steam server.
this activity puts the decrypted file in memory, temporary of course.
And completely stops the download process-- and so creates these Ups and Downs simply because it consumes a lot of activity from your SSD as well as the NIC.

Your pc also needs to decrypt steam's encrypted files while assembling them so that they can be run as video games from the hard disk, which is a different encryption from bitlocker's encryption (which will run anyway).
so your SSD/HDD is very... very busy and cannot constantly download thanks to this nonsense alone. which is why I mentioned it first.

Steams downloads are limited by

1) your CPU
2) your disk IO
3) your anti-virus
4) your ISP

Pick one