The hard part of perf work is not fixing the perf. The hard part of perf work is measuring the perf. - Sam Sacoone.
How to measure performance bottlenecks
There are a lot of tools that you can use to measure perf bottlenecks
Dev Tools - Use the timeline feature of your devtools to measure performance bottlenecks.
Read Perf Audits
Watch Talks on Web Performance
Jake Archibald's bt-conf berlin talk.
SuperCharged is a show where Paul Lewis does small performance audit on peoples' sites and recommends some fixes.
RAIL .
and a lot more.