The correlation between Tier and xpm is 0.89. The more Tier, the more xpm, and vice versa. But some values are higher than expected, some are lower. Efficiency is a difference between the value of xpm depending on Tier (blue dots) with respect to the expected value (red line).
Efficiency
Mind Breaker 121.3826
Penta-Edged Sword 84.0183
Ninja Gear 74.6883
Paladin Sword 66.4026
Elven Tunic 60.9426
Titan Sliver 53.1826
Stygian Desolator 46.1867
Grove Bow 43.3095
Pirate Hat 35.0767
Broom Handle 30.3692
Apex 30.1367
Enchanted Quiver 28.6926
Havoc Hammer 18.5983
Trickster Cloak 18.0483
Duelist Gloves 17.9092
Vampire Fangs 16.7495
Defiant Shell 16.3526
Occult Bracelet 15.8492
Ring Of Aquila 12.5495
Lance Of Pursuit 12.0192
Spark Of Courage 10.5792
Fairy'S Trinket 9.6292
Cloak Of Flames 7.5826
Mirror Shield 7.2067
Ex Machina 5.8367
Ascetic'S Cap 5.3283
Pig Pole 4.6492
Giant'S Ring 3.9467
Stormcrafter 3.8583
Arcane Ring 3.5692
Martyr'S Plate 0.7083
Vambrace -2.1805
Spell Prism -4.7117
Force Boots -5.1333
Timeless Relic -7.0317
Faded Broach -10.0908
Trusty Shovel -16.2008
Dragon Scale -17.6105
Pupil'S Gift -17.9605
Fallen Sky -19.3633
Tumbler'S Toy -27.2008
Seeds Of Serenity -28.5808
Arcanist'S Armor -32.2833
Ceremonial Robe -33.2174
Dandelion Amulet -34.7274
Quickening Charm -38.9974
Ogre Seal Totem -39.0374
Gossamer Cape -43.6205
Bullwhip -47.9305
Book Of The Dead -52.4433
Philosopher'S Stone -56.2405
Book Of Shadows -60.0933
Psychic Headband -63.2274
Telescope -65.9917
Seer Stone -65.9933
Eye Of The Vizier -75.4905