You definitely need to test on the slowest device you have. People confuse mobile devices for computers. Nothing could be further from the truth. Your overall job is to never be doing too much at one time. Vectors are expensive on CPU and bitmaps are expensive on memory. It's your job to make sure you use the best possible mix of both.
Plan all your future transitions with this in mind. Never be doing "too much at once". You've already delayed some things from happening and found it to speed things up. That course is correct.