I’ve been noticing that my Kogan Mobile connection seems consistently slower than my work One NZ SIM, even when both phones are side-by-side in the same location.
I know Kogan runs on the One NZ network, so I wasn’t expecting a huge difference, but lately it feels noticeably slower for browsing and speed tests.
Has anyone else experienced this recently? Has anyone seen any evidence that Kogan traffic is deprioritised or throttled compared with direct One NZ customers, particularly on 5G?
Interested to hear if it’s just me, a handset issue, or if others have noticed the same thing.

Maybe it's worthwhile to do some basic scientific tests and share them, rather than just speculating….? I mean, how do you even define "slow"? How long is a piece of string?
Run a Speedtest and make sure everything is controlled. Make sure you're always connecting to the same Speedtest server. Run a test against SIM #1 and do it at least 3~5 times to get an average. While doing the test, make sure nothing else on your phone is competing for the bandwidth - i.e stop your music streaming, Youtube/Netflix and torrents from downloading anything. Record the results, then repeat the same with SIM #2 - again, making sure everything is fixed/controlled. If let's say you get 300Mbps for SIM #1 and you get 280Mbps for SIM #2, sure it's "slower" but does it really matter? I guess it depends on what you're trying to do.
And then test the same thing on another phone/device.
It also depends on the plan that you're on. Some plans get throttled due to peak hours, as you might be allocated to a pool that is shared with other users.
Personally, I use Mighty Mobile, which also piggybacks on One NZ's network. I don't have any issues at all. Obviously, I'm not using Kogan and so it's not comparing apples to apples.