A 2GHz device runs at 2GHz. The processors - single or multi-core - all are synched to the clock rate of 2 GHz.

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A multi-core CPU is packaged in a singe chip, so it is more efficient - it does away with the extra support and interface structure needed with multiple processors and with the cores assembled and connected together in a single CPU chip with very small distances between each core, there isn't as much of a problem with latency and synch as might be seen in a system with multiple "single core" CPU devices.

This is much more important in modern hardware with very high clock rates than it was just a few short years ago when clock rates of a few hundred Mhz were the norm.