The comparison to a bank line is a little misleading. Tests have shown that adding a second core, or moving from 2 to 4 cores does not automatically double performance speed, or cut response time in half (depending on how you want to look at it). There are other limits on performance. The simplest one is that you have to be running 2 or 4 separate tasks to take advantage of the cores. Very little software is written to take full advantage of multiple cores. It it much harder to write a single program to do multiple things at once and get all of the timing right. And even if you can split it up, there will be points where it can only work on one cpu at a time because multiple threads are being consolidated or generated.

There is also the issue of shared resources. The whole computer has to be REdesigned to share RAM and Disk and Video access.

The bottom line is that performance improvement is not a 1:1 match to the increased number of cores. It is substantially less than that.