I agree with all what you said. I would add is not smart to run large files, specially if your DRAM is less than 1Gb.
I have a model ME033LL/A [iPad mini] and it freezes constantly with plots that have more than 2000 rows [one Y, scatter plot]. I've been able to plot larges files [5000+ rows, tree Y's and scatter plot], but zooming becomes quite slow, and at times restarts the App. This confirms your point on the dependency of DRAM memory. The ME033LL/A model comes with 512Mb if I am not mistaken.
I haven't had issues with files that are less than 1000 rows [tree Y's, scatter plot]. But as you said, different graphs have different requirements.