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.
Just download the PowerPoint Viewer and Word Viewer from Microsoft.
Unfortunately, Microsoft stopped supporting these programs. You could try Microsoft PowerPoint Mobile or Microsoft Word Mobile instead.
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
Minimum requirements
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.