Think of it in simple terms:

Two groups of machines, each looking at the same block of numbers and both groups have the same rate of finding a block. 10 slow machines that each find a block every 10 seconds, so an average of one block a second, and 1 fast machine that finds a block every second.

If the block of numbers are the same, the 10 machines only have a chance to find the tenth block, as the fast machine will have found the first 9 before the 9 slower machines. The fast machine will then start to work on the 10th block at the same time the 10th machine has 1 second left of the 10 seconds to find it.

So, what does this mean?

If you have a slow machine and are solving blocks that faster machines are also solving, then you have a very reduced chance of finding a valid block.