Is there any quick way to delete the standard emails trash files. I have had a Galaxy S3 for some 3 years and I have recently noticed that I have in excess of 45,000 emails in the standard email's trash folder.

I have tried selecting the trash folder and then using the "delete all" option but the speed of deletes is staggeringly slow, one every five to ten seconds.

It is synchronised with an Office365 account, and I have checked on the server side to see if the same folder exists but it doesn't, it is only on the phone. (Note that this is the standard email app, it is not Gmail or anything similar). 

Is there some quick way to empty the email trash folder, either through a file app, or a third party app?
Keep in mind that your phone's name for certain specialized folders, such as trash, differs from what Office 365 will call it. In Office 365's web interface, and Outlook, the folder is named Deleted Items, and they are kept in sync with your phone's comparable Trash folder. So look there to see the correlation. And yes, deleting from your phone will be dramatically slower than deleting from your computer or the web, the latter being fastest as it's connected directly to the server.