So, how fast?
To search an array of a million entries using linear search may require a million iterations of the loop.
To search an array of a million entries using binary search requires only 20 iteration!
Linear search is a linear algorithm; the time it takes is proportional in the worst case to the size of the array.
Binary search is a logarithmic algoririthm; the time it takes is proportional in the worst case to the log of the size of the array.