When I run this query, the first few records returned have a build_step_duration of 0: 

SELECT FORMAT("https://luci-milo.appspot.com/p/%s/builders/luci.fuchsia.%s/%s/b%d", builder.project, builder.bucket, builder.builder, id) AS build_url,
       builder.builder AS builder_name,
       FORMAT_TIMESTAMP("%F %X", create_time) AS create_time,
       TIMESTAMP_DIFF(step.end_time, step.start_time, SECOND) AS build_step_duration,
       step.start_time, step.end_time
FROM `cr-buildbucket.fuchsia.completed_builds_BETA` b
  CROSS JOIN UNNEST(b.steps) AS step
WHERE builder.bucket = "ci" AND
      step.name = "build" AND step.status = "SUCCESS"
      AND builder.builder = "garnet-host-linux"
ORDER BY create_Time


However, when i click on the URL, I see the step named "build" took much longer than 1 second.

This seems to be fixed for more recent builds.

Example of build with bad data:
https://luci-milo.appspot.com/p/fuchsia/builders/luci.fuchsia.ci/garnet-host-linux/b8935390485444994656

Example of build with good data:
https://luci-milo.appspot.com/p/fuchsia/builders/luci.fuchsia.ci/garnet-host-linux/b8935387024340448512