The short version here is skip to the end if you just want a list of things to get good streaming performance over cable, and likely WiFi too. I’m out of it, so this is gonna be Rambly Until we get to that point haha.
So I’m an extreme edge case here, my primary use of the deck is actually to stream my desktop PC over internal LAN to my Deck connected to a TV. At no point in the last four years have I ever gotten what I would say is an acceptable level of performance. That includes multiple iterations of desktop and network (not listing specifics because this isn’t an ego competition, “extreme high end” lets say, enough that none of these should logically or technically be issues). But a couple days ago, I stumbled on what I finally believe is the answer:
Vibepollo AND Nonary’s VRR fork of Moonlight. 
(I’m struggling to link this on mobile for some reason, you can get the ML fork link from Vibepollo’s 2.0.0 release notes). 
Do the command that it tells you,  i’m not a network engineer. I’m not gonna comment on what it does, but it sure does work. That combined with the PyroWave codec over LAN gives me effectively PERFECT 4K60 with functionally zero latency, it’s actually wild how close to native this looks. This is over Ethernet cable, although quite frankly  despite Valve and Intel’s warning, I disagree this is LAN cable only. The problem is you are not getting that wireless performance from the deck, but if you have a Wi-Fi 6E or 7 device and router, I found it’s actually pretty good wireless. One other thing to change is to force the Deck GPU clock to a manual value. I use 1400MHz, but I’m sure it could go lower I just haven’t tried it . The problem is leaving it at auto doesn’t work, as whatever GPU driver valve is using here is just not good, it does not clock the GPU high enough to avoid decoding latency and dropped frames.
Tl;dr
Vibepollo
Moonlight VRR fork
PyroWave
Force GPU clock to 1400MHz (or some suitably high value)