A lot of times these files seem to lag and be incredibly slow even if just a few dozens rows of data.
Here is how you can convert it with "gnumeric":
ssconvert file.xlsx file.csv
You may get lots of weird errors like this but the conversion seems to work fine:
(ssconvert:15244): GLib-GObject-WARNING **: g_object_set_valist: object class 'SheetObjectImage' has no property named 'style'........