First see (Xilinx Answer 15492).
If that does not help, then consider using the VCD2XAD Perl script.
VCD File Pre-processing (VCD2XAD)
XAD is a new data file that contains signal information in much more compressed format than the original VCD file. A Perl script exists (VCD2XAD) that performs this compression that will minimize file size and loading time later.
This script is located at %XILINX%/bin/%OS%/vcd2xad.pl
%XILINX% is the location of your Xilinx software installation.
%OS% is the operating system :
nt : Windows 2000/XP
sol : Solaris
lin : Linux
The conversion is done by running this Perl script on the VCD file to produce an XAD equivalent file.
xilperl vcd2xad.pl -f myvcd.vcd
This XAD file can then be loaded into XPower in place of the VCD simulation file. Since much of the processing has already been done by the XAD2VCD script, the load time will be dramatically reduced.