A microbenchmark attempts to measure the performance of a “small” bit of code. These tests are typically in the sub-millisecond range. The code being tested usually performs no I/O, or else is a test of some single, specific I/O task. 
We maintain and measure a set of such microbenchmarks for example blink_perf, dromaeo, gpu_perftests, and rasterize_and_record_micro.
Due to their nature microbenchmarks are much more sensitive to changes in the underlying abstraction levels which can result in a large number of false positive alerts. Below are some of such changes relevant to Chromium.
Automatic Feedback Directed Optimization is a process that produces profile-based optimization hints that are applied by the compiler. It can result in changes to inlining (as well as causing more inlining, it can also prevent inlining). The inlining decisions made by AFDO can have noticeable impact on micro benchmarks, especially those involving tight loops or tight recursion. Unfortunately the decisions are not always stable and can flip from inline to don't inline without any changes in the code being benchmarked.
https://crbug.com/889742 is has more details and many duped bugs.
It's also possible to make no-op changes to code, cauing the previous AFDO data to be inapplicable (e.g. function name change). This can result in apparent regressions which recover spontaneously once new AFDO data is generated based on the new code. E.g. https://crbug.com/855544 was a specific case of this. One way to dig into this is to examine the compiled functions before and after the no-op change, to see if inlining has changed.
If you have a function that you believe should always be inlined in production builds, please contact firstname.lastname@example.org for advice.
Our toolchain team regularly rolls in new versions of clang, the compiler for all of Chromium. Though it‘s rare, these rolls may cause unintended performance changes. These rolls are represented as regular CLs/commits to Chromium’s repository (e.g. https://chromium-review.googlesource.com/c/chromium/src/+/1436036), so it‘s often pretty simple to attribute a performance change to a compiler roll. If you believe a compiler roll has slowed down your microbenchmark, please reach out to whoever landed the roll for guidance. It may be difficult, but if you can reduce your test-case in any way from ‘simply build all of Chrome!’ (even by just providing the before/after assembly), it’s a huge help.