What's your preferred way of grinding threads on a DPS class? As holy priest I just join random HC dungeons as healer and I almost instant queue all day long.
I don't really have a preferred "optimal" way cuz I'm sure there are others better than me at it. But I have noticed a way which can rack up quite a hefty sum of threads especially during the Timerunner Mastery buff that increases Spool threads. Create a toon, skip the tutorial area till you are handed the cape, then do the 4 celestials which will give you 5 green spools, then head to Ordo and you will be rewarded with both Ordo achievement and the timeless isles looking for group on all raids, which will give you in total 6 green spools and 1 purple spool. After that, talk to the infinite dragon to be ported to Jade Forest, ignore the quests and just jump straight to the nearest Bazzar and get the 2 daily quests (Scenario and Dungeon). And que up heroic/scenario whichever you want, and during the waiting times I mostly either head to Sha of Anger to get the daily raid spool thread and the achievement spool then to Galleon for 4 spools in total for both Galleon and Sha. Galleon is important for the looking for group achievement that Valley of the Fourwinds needs (Galleon and Stormstout Brewry). Assuming you got both the scenario and dungeon daily done during both Galleon and Sha waiting then turn in both quests for another 2 Spools. Then I start doing the Tour Jade Forest achievements by hunting down rares which I memorized like the palm of my hands, then exploring all of Jade forest, then collecting 2 treasures. Resulting in 1 Green spool, 2 Blue spool and 1 Purple spool for the whole Tour Jade Forest Achievement. After that assuming you reached level 20, Que up all heroic dungeons and get their respective achievements. Thats like 4 green spools plus the already done Galleon which nets you the Looking for group achievements if you finished StormStout Brewry, so another 1 Blue Spool. And also finish all scenarios for 6 spools in total plus the Looking for Group Jade Forest achievement which needs (Temple of the Jade Dragon and the Greenvillage instances done), so another 1 Blue Spool.
Then I start the Valley of the Four Winds Tour achievement like Jade Forest, explore the whole area, then kill the rares and get the treasure (I stated it to be specifically 20 for that cuz the treasure is locked if youre below 20). And you got yourself 2 Green spools, 1 Blue spool and 1 purple spool.
Then I start the Karsaraang Wilds Tour, same shit gets you 2 Green spool, 1 Blue spool and 1 purple spool. Also added bonus to the Landfall rares on the alliance/horde side by killing the rares of each faction in Karsaraang. So another green spool.
So in total that is approx 25-30 Green spools, 5-6 Blue spools and 4 Purple spools in the span of like 2 hours. I'm sure of that cuz I did exactly that on a new Hunter toon. You would be surprised by how much spool threads achievements give you.
TL DR- I prefer doing Achievements as fast as I can since they give insane amount of Spools at faster rates. Like the Tour achievements, Heroic Dungeon achievements and World Raid achievements etc..