Optional Skill/Spell Speed modifier

[Lethros]

I've hit about 2.18 as a healer before (synced down), maybe 2.16, and around 2.2/2.22 if not synced. All of this was during Stormblood with level 70/mixed gear.
I expect you can still do this. Going into a synced down duty, change all your gear so that it is above the item level sync, and so that each piece (or as much as possible) has there stats such that the speed attribute is the higher/already maximum stat. When you sync down, it seems to be that the higher the item level you're syncing from, you are given bonus points to the highest secondary stat on each gear piece. While synced you can test this by switching in and out an at item level piece, and an above item level piece of the same gear slot with the same secondary stat interval/distribution. The only number in the character window that changes is the secondary attribute that is maximum on the piece (becomes a higher number with the synced piece switched in).
Look through all the various dungeons, raid tiers, etc. to see what gear options there are for speed, then take as much speed as you can get to go into a synced duty below all of their item levels.

Likewise, you can also see that if you bring an above level weapon to a level 50/60/70 duty, duties that don't have an extra item level sync, your weapons become 130/270/400, not 135/275/405. And so if you change to an xx5 weapon there's also an increase there, but not necessarily helpful if you want speed