The first question is; are the haste scales listed on Allakhazam correct? If so, I drew up a table on how haste spells apparently scale by level: I am doubting this is correct. If it were it seems to be better to downrank haste spells until the scaling catches up. Can anyone clarify if they scale from a starting level?
Not sure about haste, but slows definitely scale with level, and you'll find it ideal to use older slows that have matured over your newest slow on group level content, reserving best slows for named or raids in the name of sustain
Don't use terrible spell parsers like Allakhazam for detailed information. Quickness 1: Increase Melee Haste by 30% Alacrity 1: Increase Melee Haste by 40% Celerity 1: Increase Melee Haste by 50% Swift Like the Wind 1: Increase Melee Haste by 60%
In addition to the disagreements about Allakhazam's accuracy, I just looked at Allakhazam and the entry for Alacrity says 1: Increase Attack Speed by 22% (L21) to 62% (L37), which is not what your chart seems to indicate. Maybe ALL of these posts are wrong.