The green and grey mods dropped in usefulness a ton with the new update. First, you're spending mod energy to get them in the first place, THEN you're having to farm mod salvage with the same energy to THEN roll the dice again and hope the speed stat is the one selected to increase. THEN, the secondaries no longer have a chance of increasing each 3 levels, only once ALL secondaries are displayed. Grey mods won't increase at all unless you slice them. Then the other three secondary stats will be trash anyways. Better off getting a blue/purple.
I just farm until I get blue/purple/gold with speed already displayed then test upgrade those to 9/12. Only max them to 15 if I get a hit on speed to a decent level. +10-ish for toons I don't use often or niche teams. +15-ish for heavy hitters and arena.
Keep an eye on the mod shop for arrows with speed or key mods with +speed secondaries. I'm at the point where I'm not spending ship credits anymore so I use them on mods now.
Only grey and green mods I keep are arrows with +speed or crosses/triangles with rare primaries (CC/CD triangles, potency/tenacity crosses).