scrolling_summary.timeline_summary.json contains the summary. Open
the file with any text editor to review the information contained
within.  With a more advanced setup, you could save a summary every
time the test runs and create a graph of the results.

scrolling_timeline.timeline.json contains the complete timeline data.
Open the file using the Chrome browser’s tracing tools found at
chrome://tracing. The tracing tools provide a
convenient interface for inspecting the timeline data to discover
the source of a performance issue.

Summary example

"average_frame_build_time_millis"

4.2592592592592595

"worst_frame_build_time_millis"

21.0

"missed_frame_build_budget_count"

"average_frame_rasterizer_time_millis"