likely going to be legion greens lvl 39. i48 farmed on any toon 40+ while not in chromie time(50+ for ez)
with "of the Impatient" Stat suffix. full haste, in every slot, with haste on use trinkets if possible,
https://www.wowhead.com/item=110002/fleshrenders-meathook?bonus=6710 is one trinket,
https://www.wowhead.com/item=132963/young-drakes-impulse is another then maybe get another, or a stat stick, or 2 haste stat sticks, one being a ghost iron dragon with 2 haste cogs, maybe at 40 you can put a 3rd haste cog from MoP crafting
https://www.wowhead.com/item=102293/purified-bindings-of-immerseus will likely be of high value but ive never used it so i cant verify
at 40 you can get the heart of azeroth if someone trades you azerite gear from freehold or ataldazar, its got good haste, but for pure haste you probably wont wear any azerite armor slots other than the neck
https://worldofwarcraft.com/en-us/character/us/bleeding-hollow/intervention
https://worldofwarcraft.com/en-us/character/us/stormrage/Khloekay
^these are 20s, but this is the type of legion boe gear youre probably wanting, just scaled to 39. i48
youd be using this gem
https://www.wowhead.com/item=151583/quick-lightsphene Req ilvl 44.
there may be some heroic SoO garrosh, or mythic SoO loot, that is better than Impatient legion greens when you consider the potential to stack
https://www.wowhead.com/item=151583/quick-lightsphene. ive never done much raid gear twinking so i cant recommend anything but you should check
there will likely be certain class spells that are available at 40
one that comes to mind is
https://www.wowhead.com/spell=12472/icy-veins combined with
https://www.wowhead.com/item=120257/drums-of-fury and
https://www.wowhead.com/spell=26297/berserking or
https://www.wowhead.com/spell=92680/nimble-fingers
if you want to avoid boe farming, there is Baleful Gear tokens in tannan jungle but i dont know if this can proc sockets, they seem to be able to proc impatient suffix