Why? Download shouldn't stress the OS much.

If your issue is CPU utilization, Linux is much better than windows at prioritizing processing tasks, etc. At most Valve might need to lower the priority of the downloads to ensure other processes take priority over it (and if it's the case you can tweak this yourself before Valve does anyth8ng on their side).

If your issue is actually network throughput instead of processing, you can probably fix it by defining speed limits for downloads in steam settings and looking around the other pertine t finetuning options there... there is a fair chance that this will "feel" like a choked CPU and it might also work as an indirect control over excessive CPU load derivedfrom downloads.

If you use an HDD or pendrive or memory card instead of an SSD, a perceived slowness may be due to reaching max IO speed from that storage unit. Those examples are all typically much slower than SSDs and software might need to wait for data from disk and this can "feel" like they're running slow for CPU load reasons too.

Steam downloads are less important that what my OS does normally. Steam patching can eat up to 25% CPU. Downloads are less demanding than patching.

I don't see the problem then... neither completely hogs the OS, one doesn't really break the other...

do SteamOS patches happen while playing?