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You can try to break up the query into CTEs, and then see if any of the individual CTEs are unusually slow.

I am guessing the query is not just one select, but probably has subqueries, window functions, aggregations, joins and so on. All of these can be split into CTEs pretty easily (if you have questions about specific syntax, like "how do I move aggregation to a CTE" that's worth a separate question). Some code editors can even do that refactor automatically.

Binary search is a good approach here. Start by splitting off half your query into a CTE, and the rest as the final query. See if the CTE is much faster. If so, then extract another half of the remaining query as a second CTE. If not, then extract half of the CTE instead. If you repeat this it should eventually identify the specific part that is slowing things down.