A Lightning Web Component on a record page is slow and occasionally shows fields the user should not see. How would you investigate and fix it?

For visibility, I would check the Apex controller: it should enforce sharing with the with sharing keyword and enforce object and field permissions, for example with user mode database operations or security stripping of inaccessible fields. Using Lightning Data Service where possible also respects field-level security automatically.

For performance, I would check for repeated server calls, unnecessary data being returned, missing cacheable annotations, and large rendering loops. I would use browser developer tools and debug logs to find the slow part, then reduce payloads, cache read-only calls and load data lazily.

Interviewers want awareness that Apex runs in system context by default and must enforce CRUD, FLS and sharing explicitly. Good candidates combine security and performance diagnosis with specific tools. A pitfall is assuming the UI hides data so security is fine.