CSES standard solution for range query problem giving TLE
You are already using the optimal solution algorithm for the problem. A few things, which might speed-up the computations (.split should not be the limiting factor in most likeliness)
Variable naming
Use names which match the problem description, so that it becomes easier to follow your code. So, instead of x, y it'd be a, b. Also, in python, it is a good practice to name your variables in lower_snake_case.
sum is an builtin function. Do not override this.
Computing len
You do not need to iterate over indices as the index is not really being used anyway. Lists can be iterated over their values itself.
Lazy iterators
Instead of generating list inline, you can use generators so that the values are yielded when iterating, and not consuming memory.
.split
Not passing a parameter to .split is same as providing whitespace as parameter.

Rewrite:
def solve():
    n, q = map(int, input().split())
    arr = map(int, input().split())
 
    cumulative_summation = [0]
    running_sum = 0
    # Not calculating `len` of `arr`, also, you don't need to calculate it.
    # the length is known to be `n`.
    for value in arr:
        running_sum += value
        cumulative_summation.append(running_sum)
 
    for i in range(q):
        a, b = map(int, input().split())
        print(cumulative_summation[b] - cumulative_summation[a - 1])