Nepomuk is a pig. It's a research project that isn't ready for field trials. Fork the tree from it and work in parallel until it is fully core aware with OpenCL on both the CPU and GPGPU, then put it back in, but keep it optional for folks on older systems and weak GPGPUs.
The CPU usage is usually not the problem. Memory can be, but if your system gets slow due to nepomuk/strigi it is more likely it is disk access. Especially if your home directory is encrypted.