So you can tell if you are using extracts (.hyper) or live by looking at the visual cue in the data section
in this image the top one is Live (single barrel) and the bottom one is an extract (double barrel)
To change your .twbx to a .twb just do a save as and select .twb. In the file location you'll now see an extra folder with the datasource (either Excel or hyper files) in. This is because a .twbx is just a zip file of the workbook (.twb) and the datasources, and when you re-save it as a .twb, Tableau unpacks the datasources into this folder.
and yes .hyper files will be much fast than live against excel, as the extract is optimised for Tableau (the ,hyper files are also much smaller than the original excel).
Hope that helps