A few weeks ago sbt version 1.0.0 has been released. One of the big ticket items in this release has been an improved incremental compiler that needs to compile less and therefore is faster. Slow builds are a common complaint for Scala developers so this generated a lot of excitement.

In reality however, build times in version 1.0.0 have not seen any improvement but rather a significant slow down. While the incremental compiler itself is faster, other areas of sbt have seen a sudden worseing of performance. This has been reflected in a flurry of Github tickets and discussions on the Gitter channel.

The sbt core developers have been understandably reluctant to merge in spurious pull requests by the community without evidence that they really improve performance beyond a simple micro benchmark.

The Scala compiler, for example, has a solid performance benchmark suite that is run on every commit and gives immediate feedback about proposed changes.

Something along these lines is needed for sbt, too.

Since there doesn’t seem to be anything around yet, I took upon me to write one.


I took an sbt project provided by Sam Halliday that had pathologically bad performance.

The problem with this project is its very large classpath and many submodules - it contains almost no source code itself! Before compilation even starts a large amount of time is spent on checking if classpath entries have changed.

Around this test repository wrote a little bit of tooling for executing and timing sbt commands and running them with various sbt versions.

The benchmark sets sbt up to download all the required dependencies before the actual task under benchmark in order to ensure fairness. It then benchmark tasks with various sbt versions and measures execution times.


The timing results are as follows:

command sbt 0.13.16 sbt 1.0.3
startup 37 seconds 45 seconds
compile once 89 seconds 129 seconds
compile twice 107 seconds 189 seconds

The results are clear: sbt 1.0.0 has seen a substantial slowdown - at least for this particular use case.

In case you’re wondering, these benchmarks were run on a 2015 Macbook Pro with a 2.9 GHz Intel Core i5 and 16 GB of RAM.

Future work

I’m planning on tracking and comparing the performance of future sbt versions. In particular, 1.0.4 is going to be released very soon and has some performance improvement patches.

The benchmark can also be extended with more test case to give insights into a wide range of performance scenarios. Contributions are very welcome!