You could make an argument that vs weaker targets, where moving and attacking quickly is sufficient, then speed still have decent synergy with earthquake. And it falls off once you stay stationary and attempt to spam it (then you lose shockwaves and your combined dps goes down).
I'd say main issue of the build is scaling of single target ability - sor. It has negative synergy against fast moving/teleporting enemies, being slightly delayed and requiring the target to sit inside the aoe in order to receive full damage.
Then mix in the cooldown, which pormotes really annoying gameplay of spamming other attack in between SOR. And the shorter sor cooldown is, the less efficient your filler attacks are going to be, you will miss frames. That and fact such gameplay is just annoying.
I'd say two routes would attempt to solve these issues:
- make SOR spammable, nerf it accordingly, but also set final power of it higher than current in the endgame, so it is more capable
- increase the cooldown of SOR, again increase its power accordingly, this gives time to more comfortably spam earthquake in between SOR usage, every 4-5s for example.
2nd solution however poses problem that was mentioned before, continous spam of earthquake against beefy target is largely diminished due to overlapping aftershocks.
Bottom line is, build would benefit from more single target potential, and less annoying gameplay in form of juggling 2s cooldown main skill.
Also looking back at the old iteration of build, way back to Ultimative days, it was more interesting, more skills were involved, Fortress was viable part of the build, added flexibility. It would be nice if these variants still had place in the game, and build gameplay would provide more variety and skills, rather than just spamming 1 or at most 2 abilities.