Add latency debugging guide

Add documentation for how to debug latency / slow queries.

Topics:
1. Do not use the first query as a measurement
2. Use (end-to-end) tracing
3. Use the built-in gorm logging function

Start by locating the repository's documentation and references to the built-in GORM logging function. Write a latency and slow-query debugging guide covering the three listed topics, including end-to-end tracing, and consider the guide done when each topic is documented clearly.

Written by the indexing model from the issue text.