When a processor speed is given for a multi-core processor, is that the cumulative speed or the speed of the individual cores?
Frequency is not cumulative. The chip runs at 1.8 GHz, each of the cores runs at 1.8 GHz, all four cores run at 1.8 GHz. If one car travels at 100 km/h and another car runs beside it at 100 km/h, you just have two cars running at 100 km/h, you don't have anything running at 200 km/h.

Instruction execution rate, on the other hand, is cumulative. If a single-core processor running f GHz can execute k instructions per second, then an n-core processor running at f GHz can execute n ⨉ k instructions per second*, just as our two cars together carry twice as many people as one car.

*In practice you won't see an n-fold increase in speed, as some resources are consumed in co-ordinating the cores.