Downsides to using certain QoS rules?

[Daeron]

Did anyone research how the existence of certain QoS rules may affect performance? If so, how much do they affect and in what ways?

For example simple port matching vs connection bytes reach vs packet length. Which one uses more CPU, more RAM or both? I understand that L7 pattern matching is expensive and is generally unreliable so there's no point turning it on.

Are connection bytes/packet length something that the router always keeps track of (therefore cheap/free in terms of performance), or only if these kind of rules are turned on? Does including something like TCP only besides port 80 help vs only matching on port 80 in terms of performance, or does it potentially even make it worse?

I had a "maximum packet length <512" rule that many people on these forums recommend. I also currently have a second one with ~1300 since it appeared to me that certain games can have larger packet sizes when enough players are on the server. Does this result in twice the theoretical performance loss, or only one?

On top of that I was experimenting with the default settings of matching ports 80 and 443 and using the connection bytes reach field to give regular browsing priority over downloads/streaming videos. Does this put a noticable stress on the CPU/up the memory usage?

I have a TL-WR1034ND v1.x router and a 30/3mb connection. I suspect even if it does my internet speed is most likely too slow to hit the limits of the hardware. Still, I would like to know how much these settings generally affect performance or maximum throughput.

Also, does the minimum BW field have any purpose besides when using ACC (assuming the % BW is also filled with an equivalent value)? Sometimes my internet gets unstable, specifically my upload speed, so I was wondering if it could be used to give my gaming class priority before the rest of the bandwidth is allocated. But I suspect if my upload bandwidth falls under the specified global limit, QoS stops working either way. And ACC can't adjust my upload speed, so it does not appear to be useful in that case.

[Lantis]

Like any computer, the more you ask of it, the longer it takes (even if this is measured in milliseconds)
If you give it a list of 100 match criteria, and the first match sticks, it's very quick.
If it doesn't match until the final one, that's slower.

Ultimately, i don't think you'll notice a massive difference on a reasonable router.