Well, most of that data is video, so it won’t be needed all at once. What BD does have, far more than DVD, is programs. All those fancy menus and interactive features are programs (written, I believe, in a flavour of Java). The disc player needs to load and run them. I suspect that cheap BD players skimp on their internal CPU, memory, etc, that runs these programs, as well as on the mechanical parts, so the programs load more slowly, run more slowly, and need to request more data from the disc more often… over and above the job of decoding video and audio and making it available to the outputs.
DVD players have a capability to run programs as well, but they aren’t in Java and aren’t nearly as complex.