read intel's documents carefully. I believe Xeon, like Pentium Pro, may be optomized for "32 bit clean" code. (IE, WinNT4 and Linux) If that's the case it will actually perform the same or maybe a bit more poorly than the PII & PIII processors when running Win9X.
The reason I believe this is Xeon and the now extinct Pentium Pro are targeted at the "Server" market which all run Netware, WinNT, or Unix/Linux. These are all 32 bit "clean" operating systems. Win9x still contains allot of 16 bit code. There is a "mode shift" in the processor each time you change from one to the other. PII and PIII may be built to accomodate this better than Xeon. Like I said, check Intel documentation.
