Title says most of it. I use the feature a lot during development, typically through Ctrl+Click or CMD+Click (on macOS) on a symbol.
However more often than not, the feature does not produce a result in a reasonable time frame, especially when the Symbol Under Cursor is defined in a file that isn't currently open.
Sometimes, after some seconds, the Symbol is actually followed and the editor content changes accordingly but usually at that point I have already moved on, no longer expecting any jumps in the code. Also, there is no visual indication that the system is doing anything in the background and that I should be expecting a result.
Sometimes the feature works ok (~1 sec delay), sometimes it doesn't work at all (no result) and sometimes it works badly (several secs. delay) but there's no way to tell beforehand.
I remember this function working really well and also more or less immediately in the late 2.x and early 3.x releases of Qt Creator. Since then it feels like it has degraded in performance quite a bit, now to the point where I feel it justifies a bug report.