PC Win8.1 64bit 16GB RAM: I use LO 4.3.1. I have a 70kb XLSX file which needs 50 seconds to open, the same file as XLS just needs 5 seconds

I do accounting with Excel but my office mates use LibreOffice. It takes them about 50 seconds to open my XLSX file with 70kb. If I save it as XLS the file gets bigger (170kb) but LO Calc opens it in 5 seconds. The file has 6 sheets, data in 350 rows and 30 columns. About 200 cells contain formulas.

Switch to LibreOffice or save as .xls

I only can emphasize what @karolus wrote above. Change to LibO and be happy!!! Many of my friends and collaborators did so; they are happy too.

Well what is your question?