Yes, Zen has been getting progressively worse and it has been impacting Zen Wholesale customers too of late. Latency and weird behaviour generally and throughput issues on single threads come back from time to time.
I'm on Zen FTTC with BTw backhaul.
My BQM looks OK as do my Ookla speedtests, but in the last week or so I've noticed a lot of buffering when using streaming services. Assumed it was something my end although I've not figured out what. I've flashed the stock Fritzbox 7530 firmware back on instead of OpenWRT to exclude that as the culprit.