If you're not sure where to start and don't know what resource type is your bottleneck such as memory or CPU, one place may be to do some analysis on your server's wait stats. Here's a very good article that talks about that:
https://www.sqlskills.com/blogs/paul/wait-statistics-or-please-tell-me-where-it-hurts/
If you know what resource is typically the bottleneck and just want to get to what queries are the problems, you can look at the dynamic management view sys.dm_exec_query_stats.
https://www.sqlshack.com/searching-the-sql-server-query-plan-cache/
this system view shows you the reads, writes, cpu time, and total duration of the queries running on the instance that are in the plan cache still.