A report, a which is seen as a table is very similar to the excel layout. The Export Engine exports the data row by row.
A document, which has more objects such as images, then a table does. The Export Engine fetches all the object’s content, calculates their layout and exports them into a specific place in excel.
So, exporting a report and a document have different logic in generating excel files, and their performance is different.
A "Client request time out" usually means that the client sends a request to an intelligence server and it fails to get a response for a long time.
In a typical case, no matter if you are exporting a report or a document, the intelligence server creates a job which has an export task. The Client will send a request to fetch the job status until the job finishes. When the job finishes, the client sends a request to get the exported result. Fetching the job status or getting the exported result should be fast and it is a rare case that the client request times out.
