Matt Terwilliger

What I’ve learned about performance engineering [teams]

Over the past year or so, I’ve had the opportunity to work on performance engineering at Mailchimp. Performance is an important feature for any web application. It’s been shown to impact both customer retention and conversion.

I find performance engineering to be a fascinatingly vast problem space, both from a human and a technical perspective. Mailchimp is (for the most part) a monolithic application, incepted a decade ago. The company and the features we offer have grown a lot since then: today we have several hundred engineers across dozens of teams contributing daily to the same shared codebase. Ensuring our application’s performance across this large surface area continues to prove challenging. My work has mainly centered around improving the backend performance of our monolith. This post outlines a few methodologies I’ve found effective for doing so; hopefully some of the ideas are transferable to other environments.


If you’re starting a performance engineering team from scratch like we were, one of the first (and most daunting) questions is simply where to start. Maybe you have some direction from leadership, some data from research and support, and telemetry around your application itself. It’s easy to get caught up in this stage – don’t.

You will never establish a total ordering of performance issues in your application. The best thing you can do for your customers and company is to establish a framework that enables you to make locally optimal decisions based on the information available to you and the constraints at a given time. I recommend starting with something like this:

A framework for pragmatic performance engineering

When I first wrote this, I omitted an important step zero: observability. This framework presumes some level of telemetry in your stack. The ability to measure the impact of an issue and the impact of your fix is critical; you need some means to extract and aggregate arbitrary timeseries data from your application. I also recommend that you invest some time trivializing the process to profile (or trace) in production. Make sure you can find what is slow at a high level, for whom, how slow it is, when it’s slow (always? 50% of the time?), and why. With these tools in your tool box, let’s get started!


The first step is to examine the data available to you, both qualitative and quantitative. Everything from load balancer logs, to Google Analytics, to support data, and bug reports provide valuable context.

Next, collate the raw data into a story for each performance issue. Some questions to consider:

Start building this story in the ticketing system of your choice.

Taking the time to frame each performance opportunity into a cohesive and compelling narrative is worth it. These tickets aren’t for just you, they’re for stakeholders across the organization. When it’s time to give an update to leadership, or talk about the impact you’ve made at an all-hands, you’ll be thankful that the details of “why did we start looking at this darn thing in the first place?” are captured for posterity. When a support agent hops into channel with an issue, it’s nice to say “yes, this is on our radar, x other customers are experiencing the same pain, and you can follow along here.”


At this point, you have an unprioritized backlog of stories. Super high-level stuff with limited information as to what technical factors contribute to the performance issue. Perhaps some tickets already jump out: high-latency, high-traffic pages, slow, mission-critical jobs, or web requests consistently timing out. Great. In order to decide what to work on first, next, you need to pop open the hood: it’s time to break out the profiler and leverage your telemetry.

Your goal at this stage is not to solve the issue, but to gauge what effort may be required in a fix. Timeboxing is your best friend here. For each issue, spend an hour or two digging. You may need to take a staged approach: first, attempt to answer “what makes x slow in this particular instance?” Reproducing a performance issue can sometimes be tricky. Perhaps there’s caching involved. Perhaps the operation that’s slow has side effects so you cannot re-run it at will. Regardless, before you fix a problem, you need to pinpoint it.

Once you’ve reproduced a performance issue once, you’re not done. Don’t run and make the fix yet! You cannot extrapolate based on a single observation of slowness. I.e., if one database query made one web request slow for one user, that does not imply this same database query is responsible for the poor performance of an endpoint in aggregate.

What you have now is a hypothesis. “I believe this database query causes the poor performance of this web endpoint for these x users.” We should now attempt to [dis]prove the hypothesis. Add instrumentation around the database query. Switch gears for a bit to allow your metrics to stream in. If this new data confirms your hypothesis, congratulations — you’ve taken a vague report of slowness and broken it down into an isolated component whose impact you can now quantify. You’re in a very, very good place. If the new data disproves your hypothesis, don’t despair! You’ve validated why this step is important. I’d rather invest a couple hours of investigation up front than a week of development work optimizing something that will have a negligible impact.

Continue to fill out the story in your ticket with new information you gather at this stage. Depending on the scope of the problem, you may also find it helpful to break out isolated fixes into their own tickets.


Let’s step back for a minute. First, we transformed raw performance data into a story. Next, we spent a couple hours investigating each story. We’ve now navigated our complex system and isolated a particular component that causes a measurable slowdown in a known context. Y’all, that was the hard part. Now comes the fun stuff.

How you sort your issues is up to you; at this stage you have all the information necessary to inform prioritization. You understand the who, what, when, where, why, and how for each issue. Go forth and write code! Don’t forget to tie your story in a bow by measuring and documenting the impact of your improvements along the way.


This is a fluid cycle. Over time, you’ll fix issues and reports of new ones will surface. As you establish a process that works for your organization, and improve tooling, etc., you can begin to share the wonderful world of performance engineering across your company. I find this, establishing a culture of performance, to be an ongoing challenge. I’m excited by the strides we’ve already made and for what lies ahead. If you found this interesting and have thoughts or questions, I’d love to chat. Thanks!

Some additional considerations