George Ou compares Microsoft Office 2007 to OpenOffice 2.2 in memory and CPU usage using the OOXML and ODF file formats. The conclusion according to Ou:
"We can see that the OpenOffice.org ODF XML parser (while vastly improved) is still about 5 times slower than Microsoft's OOXML parser. OpenOffice.org also seems to consume nearly 4 times the amount of RAM to hold the same data. While OpenOffice.org continues to have fewer features than Microsoft Office, it continues to consume far more resources than Microsoft."
Read the article at zdnet.com