Reading a slow page
Start a slow page from the network panel, not from a hunch about React.
When a page feels late, the first question is where the time went. Open the network panel and sort by duration. Three shapes show up again and again.
One request dominates. The server is doing too much for that URL. Read the query plan, the serializer, and anything that runs once per row. N+1 queries hide here. So do endpoints that build a whole report when the screen needs twenty lines.
Many small requests. The page is chatty. Each one is fast and the sum is not. Batch them, or return the list with the fields the first screen actually renders. A waterfall of "then fetch the author, then fetch the counts" is a design problem.
The requests are fine and the page is still late. Then it is the client. A large bundle, a chart that measures the whole document, or a list that renders thousands of nodes. The performance panel will show a long task after the responses arrive.
Guessing "it's React" or "it's the database" skips this sort. The panel is boring and it is usually enough.