I recently traveled in Japan and found the O2 data was so slow it was unusable even though I had set up the Travel bolt on, at £6.00 per day. Map apps would take many minutes to load and web searching was all but impossible. After a few days of this I researched and saw that O2 throttle the speeds making it slower to use. I also found that an e-sim is a much better option. After a pretty simple set up and for a fraction of the cost (data only so need to use WhatsApp, FaceTime, Skype or such for calls) it was great. Importantly the speeds were back to normal, and fast. O2 need to be honest and up front about this.