MS Office, LibreOffice are too heavy and require a few moments to launch.
Is there any program starting and responding very quickly?
Since you only want to view the files and not edit them... there is a free software offered by Microsoft itself for just viewing these files.
I didn't test myself, but I strongly assume that these viewers are faster than a full office suite with editing possibilities so it should start faster.
Perhaps you could take a look at Kingsoft Office Suite 2016 Free (WPS v10.xxx) (my friend uses that).
WPS Office 2016 Free is a powerful office suite which includes a word processor, a spreadsheet program and a presentation maker. These three programs can help you easily deal with office tasks
Required Operating System:
Comparing to other mentioned softwares, it uses much lesser resources e.g RAM, CPU comparing to rivals like Ms Office and worth a try.