I was successful in solving a challenge in codility, but my solution failed performance tests. How can I improve my solution?

**Challenge:**

Integers K, M and a non-empty array A consisting of N integers, not bigger than M, are given.

The leader of the array is a value that occurs in more than half of the elements of the array, and the segment of the array is a sequence of consecutive elements of the array.

You can modify A by choosing exactly one segment of length K and increasing by 1 every element within that segment.

The goal is to find all of the numbers that may become a leader after performing exactly one array modification as described above.

Write a function:

def solution(K, M, A)

that, given integers K and M and an array A consisting of N integers, returns an array of all numbers that can become a leader, after increasing by 1 every element of exactly one segment of A of length K. The returned array should be sorted in ascending order, and if there is no number that can become a leader, you should return an empty array. Moreover, if there are multiple ways of choosing a segment to turn some number into a leader, then this particular number should appear in an output array only once.

For example, given integers K = 3, M = 5 and the following array A:

```
 A[0] = 2
 A[1] = 1
 A[2] = 3
 A[3] = 1
 A[4] = 2
 A[5] = 2
 A[6] = 3
```

the function should return [2, 3]. If we choose segment A[1], A[2], A[3] then we get the following array A:

```
 A[0] = 2
 A[1] = 2
 A[2] = 4
 A[3] = 2
 A[4] = 2
 A[5] = 2
 A[6] = 3
```

and 2 is the leader of this array. If we choose A[3], A[4], A[5] then A will appear as follows:

```
 A[0] = 2
 A[1] = 1
 A[2] = 3
 A[3] = 2
 A[4] = 3
 A[5] = 3
 A[6] = 3
```

and 3 will be the leader.

And, for example, given integers K = 4, M = 2 and the following array:

```
 A[0] = 1
 A[1] = 2
 A[2] = 2
 A[3] = 1
 A[4] = 2
```

the function should return [2, 3], because choosing a segment A[0], A[1], A[2], A[3] and A[1], A[2], A[3], A[4] turns 2 and 3 into the leaders, respectively.

Write an efficient algorithm for the following assumptions:

```
N and M are integers within the range [1..100,000];
K is an integer within the range [1..N];
each element of array A is an integer within the range [1..M].
```

**My Solution**


 def modify(segment):
 return [e+1 for e in segment]
 
 def dominant(A):
 d = dict()
 lenOfHalfA = int(len(A)/2)
 domList = []
 
 for i in A:
 if not i in d:
 d[i] = 1
 else:
 d[i] = d[i]+1
 
 for key, value in d.items():
 if value > lenOfHalfA:
 domList.append(key)
 
 return domList
 

 def solution(K, M, A):
 # write your code in Python 3.6
 dominantList = []
 
 x = 0
 while x <= len(A) - K:
 modifiedA = A[:]
 modifiedA[x:K+x] = modify(A[x:K+x])
 dominantList += dominant(modifiedA)
 x += 1
 
 return list(set(dominantList))