Hi @rijash,
Thanks for reaching out.
The span Requesting and receiving the document is created automatically by the agent using the Navigation timing API. It includes the time for downloading the initial html document. Specifically it shows the time between performance.timing.requestStart and performance.timing.responseEnd. There could be a few reasons why downloading the initial document might take a long time, one possibility could be that the html document is being generated dynamically and it takes longer than expected.
I've also created this issue to provide more documentation in regards to automatically generated spans!
Cheers,
Hamid