If you only ever do one thing at a time, the speed will be about the same.
Go to the CPU tab above and look at the old A64 vs. A64 X2 and P4 vs. P4D articles, the relative speedups will be similar, comparing 2 CPUs with same clock and cache, one single-core and one dual.
The main benefit is doing 2 CPU-taxing things at once, both tasks run at full speed.