The trick to performance management of software engineering, whether you're in a small startup or a large organization, is to remember that software engineers' performance can almost never be measured in terms of "how much." Your most productive engineer could be the person who thinks about a problem for three days, writes two lines of code, and then you never see an error or hear about those lines of code again because they just work.
More to the point, most software engineers chose this career because software engineering is something they enjoy doing. (If your software engineers aren't people who would write code even if they didn't have to for a living, then you're hiring wrong.) They don't need to be motivated to produce *something*.
Rather, performance measurement of software engineering is about *what kind* of work the engineer is doing. Is what they're producing what will get you to a Minimum Viable Product, or did they go off and wander into something that's more along their latest interests? For starters, you need to have that MVP defined. That's not something engineers can do -- not alone, anyhow, even if we have a ton of experience and knowledge in the domain, because we always get drawn to what interests ourselves rather than what's best for the company.
So that's how you evaluate performance: Are they working on things that are building that MVP, or are they working on other things? Are they aware of how much technical debt you'll tolerate? Greatly exceeding that can be just as bad as introducing too much.
If you're following an agile method, this is going to be easy to track, because you're going to be involved in this process, interacting with the engineers on a regular basis and seeing what they've done, what customer-facing user stories they're using. You're going to have exposed for you who is constantly understating the amount of effort they need or overstating it. Because that's what scrum and agile do: They don't solve your problems, but they expose them, so that you can see what issues are there.