NI offers 2 DSA (Dynamic Signal Analyzer) devices with hardware signal processing capabilities. Calculating cross-power spectra is usually considerably faster when using these devices.
One of these devices, the 4551, has two channels of analog input. It will allow you to perform one cross power calculation. The other device, the 4552, has 4 analog input channels allowing for 2 cross-power spectra. Both DSA boards offer simultaneous smapling, hardware anti-alias filtering, and 16 bits of resolution.
If speed is a major factor, one possible workaround with your hardware would be to log the time-domain data to file and perform the cross spectrum calculations later off-line.