Bad performance when open a bi publisher report in excel

 We use bi publisher(xml publisher) to create a customized report. For a small report, user like it very much. But for a bigger report, users complain about the performance when they open the file.

I know it is not a native excel file, that may cause the bad performance. So I ask my user to save it to a new file as a native excel format. The new file still worse than a normal excel file when we open it.

I did a test. We try to save a bi publish report to excel format, the size shrink to 4Mb. But if we "copy all" and "Paste Special" value only to a new excel file, the size is only 1Mb.