Let's put it this way. ALL speed additions come from crew power, not ship upgrades.
Ship speeds gets at least + 60 for the vast majority of ships, including CapShips.
Is a zeta worth 1/60th of the total crew power? As it turns out, it's very very close.
Therefore 1 zeta gets you +1 speed except in a few corner cases. In those corner cases when you're very, very close to your next +1 and you're zeta'ing the pilot of a particularly fast ship, it's possible to get +2 from a single zeta. On the other hand, with slower ships if you're particularly far away from the next +1 speed it's theoretically possible to get +0.
That said, if you get +0 from a zeta, you've put yourself incredibly close to the next plus one speed and upgrading a couple mods to 6-Dot or adding just a single gear piece should put you over the top.
So... is it worth it to zeta a pilot just for ship speed?
As it turns out, head to head matchups with Negotiator can easily turn on which Nego gets to AoE first (because they get assists from their ships while Dazing the other side which then can't assist the enemy Nego and thus severely limits opposing damage). Sure, it's possible to come back from going 2nd, but you're much better off going first. So taking GK all the way to r7 and adding the zeta and giving him a full complement of 6-Dot mods is sure as heck worth it because it's not uncommon for your victory to turn on your Nego's ship speed.
On the other hand, if your pilot is g12 and you're not interested in that pilot's zeta, there's PLENTY of room for speed improvement without dropping the zeta.
In general, I find CapShips you plan to run maxed in head-to-head matches are worth a zeta that doesn't help you in squad much (Holdo, for instance). Other ships, especially reinforcements and/or ships that can gain TurnMeter aren't worth a zeta strictly for ship performance.
And, yes, I know that it affects more than speed, but turn order is still more important than 3% more health & protection Whatever you get from adding a zeta, I seriously doubt it's a full 3%.