Is this expected behavior of Cake with a download happening?

Before I start I would just like to say I am using a Cake implementation via Asus merlin firmware but I can configure all options if needed so should be somewhat relevant. I don't know if this is a bug or not. Download and upload values are really lenient as is 40/9.84 (usually 45.5 and 11.3 without any qos).

Here is a normal waveform result when nothing is going on:

If I could have this all the time then no issues. I was downloading the overwatch 2 season 3 update and I limited myself to ~20Mbps in the blizzard launcher. I thought to myself I'll play street fighter online while I wait... bad idea. It became apparent very soon that I was having issues that I don't really understand. A waveform as the overwatch 2 update happened:

I get the 4k video streaming since download went down but I have much worse jitter. I don't get that at all. How can saturating the bandwidth in the very first result = 1 ms jitter but running a download at half the rate of my connection and letting waveform use the other is somehow causing higher jitter. Both the overwatch 2 download and waveform were performed on 2 different pc's both wired in but the same issue happens if I just have 1 pc doing the waveform and the download.

Is there some setting I've missed in cake or is this some ISP issue I can't fix? If it's ISP why is a single pc able to max my bandwidth and not show any issues but as soon as another pc takes some bandwidth it's just very bad?

Thank you

What is the ram and cpu utilisation on the router when the unwanted behaviour occurs?

And what device is it?

Oh and of course, if your device isn’t running openwrt there’s not a lot people here can help with. Merlin has its own forum

I think I figured it out anyway... I don't know if this is some bug in cake or not but the solution is to not rate limit a download. Just go full out. Whenever I limit (even to 10Mbps) it just completely messes up the jitter for some reason. Some examples:

Downloading linux iso in chrome from ubuntu website with 0 limiting (I don't even know how to rate limit chrome lol):

No limit OW2 blizzard download launcher:

Almost perfect now. Didn't get low latency gaming ticks because >10Mbps download needed for low latency gaming according to FAQ.

Capping blizzard launcher to 20Mbps:

Back to jitter heaven here.

Capping blizzard launcher to 10Mbps:

Arguably worse than 20 Mbps limit (insane 95th percentile readings compared to 20Mbps).

Well glad I figured that out. Cake likes no limit.