PS late reply,
Here's my experience in Japan with Circles Jetpac on Softbank network.
1. Tokyo, is primarily ok, usable speed, but it's nowhere near 4G speed despite getting 5G signal.
2. Kawaguchiko Area (Mt Fuji Lake), Lake Saiko, Lake Motosu, Yamanashi Prefecture - Speed terrible. I had an e-ticket that I needed to launch the website and let the staff interact with it (I cannot use screenshots or pre-load page). I have to wait quite a while for a webpage to load text only.
3. Nagasaki, Kumamoto, Kagoshima, Beppu, YuFuin & Miyazaki Prefecture - Speed is extremely slow, even in town areas. I had to use WiFi in some establishments to get things done.
4. Fukuoka, usable speed, similar experience as in Tokyo
In conclusion, Major big cities are still ok. In smaller towns and the countryside, you get a good signal but sometimes not usable.
I belive it's network prioritization in play (Throttled by the network provider), I previously used a local SIM card on SoftBank, experience wasn't like this.
Tested on iPhone 12 Pro Max & 14 Pro Max on iOS 16.3.1
If speed is critical, get a local SIM card.