I get the "Request was taking too long.  We're sorry, we couldn't finish what you asked us to do because it was taking too long." message when trying to open a 9.14 MB xlsx file.

Hi,

Looking at your concern, I think you are working SharePoint custom code application. The issue comes out when the proxy is taking much time. Though your question is not very elaborative, please let  us know what is that you are working on. ( Is it you have developed your code ???,  Is that you are using some existing custom document library, what kind of Site do you have ???

If you are using Custom Code to open file, you need to change your code to this following lines.

SPUserCodeService.Local.WorkerProcessExecutionTimeout = 40;
SPUserCodeService.Local.Update();

Refer to this site : Time out problem

Cheers!!!