It's the same idea at its core, but Fathom aims to be a tad more opinionated & simpler to use. Also Piwik doesn't scale that well for heavy-traffic websites without throwing a lot of hardware at it.
Fathom is built in Go and only tracking aggregates, allowing it to run on surprisingly little beef. Benchmarks have us handling up to 20.000 pageviews per second on a single-core server while using less than 100MB of RAM.
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