Consider the array sorted in decreasing order, and it's $$$k^{th}$$$ element speed as $$$x$$$.

As it clearly produces $$$k$$$ indices $$$[0,k-1]$$$ which have speed greater than or equal to answer. there is no reason to consider any value lower than this.

Following, If we increase the speed further, The computer at $$$k^{th}$$$ position will not be able to operate and hence total number of computers will be fall less then $$$k$$$.

Hence, $$$x$$$ is the optimal choice.