I never learned everything off by heart. Look in several aircraft guide books, there will be different speeds for the same model. I found it more important to have an appreciation of what kinds of speeds each aircraft type was capable of. Eg. Most jets 400-500kts, turbo props 200-300kts then smaller things going around at 100kts ish. Likewise with typical routes and passenger numbers. It may have changed, but I never got asked in tests or interview to give a precise figure for the cruising speed of a 747 for example. Learn them by all means but just don't panic if you can't remember every figure.