Well, a weekend wasted.
Summary: assuming my phone is not just faulty (everything else is fine). The problem feels like the S21 4G modem is much more sensitive than the S20's and in a bad way; I hope it can just be better calibrated.
You might not notice this issue in city areas (strong signal areas) or where you are using WiFi (wait until Spring for post-covid wave 2/3/4/5 walks!) but the 4G upload speed is being impeded by something (and the stats from the S21 compared to the S10 are shockingly poor).
The download speed is generally ok outdoor (not impacted as much) but you'll find the the modem is more temperamental generally, which means slower 4G with the S21 than with the S10 or S20. The phone is capable though as in good conditions, I reached 4G 330mbps down and 75mbps up in one location. You just wouldn't get these freak stats if the phone wasn't capable or faulty (i.e. flawed beyond rectification).
I've now tried a factory reset and different networks (EE and O2) - the same issue presents. At first, I thought it was resolved after a factory reset because I left WiFi on (which appears to have some effect on the stats being reported although maybe not actual user experience)... Strangely if you leave WiFi on but not connected to a network, the latency problem is masked to some extent. Doesn't give me a lot of confidence in Samsung's coding.
I really hope this means it's a software issue but I'm not convinced.
Tested: throughput, latency, signal strength, location from cells (indoor worse than normal, outdoor seems as I'd expect - which is making me think the modem is too sensitive; I've tried physically hopping over cell locations but I'm reluctant to believe I just have a cluster of neighbouring bad cells), network, settings, any apps that might 'interfere' - i.e. have some kind of impact on the connection, safe mode.
Ruled out: network, cases, factory reset, apps, phone settings.
Complicating factors: each connection to a cell is somewhat unique, cells can be poorly performing yet it's impossible to find out offically from network providers, experience from one day to the next can be volatile in busy areas.
Conclusion: will wait for software updates (i.e. I don't just have a faulty phone) but need Samsung to acknowledge there is an issue really. Problem persists in firmware B6/B7.
