It's because you have a recursive function where you are passing the vector into the recursive calls like this:

class Solution{
public:
    long long  mxm(int arr[],int i,int j,vector<vector<long long>>t){
        // snip...
        left=mxm(arr,i,k,t);
        // snip...
        right=mxm(arr,k+1,j,t);
        // more stuff...
    }

If you do it like this, the vector gets passed by value, i.e. copied for every function call (instead of passing the same vector object, it makes a new, identical one), unlike some other programming languages like Java. This can get pretty expensive. Worse yet, since you are using this vector for memoization, this means that the memoization actually has no effect. Lines like t[i][j]=mn; do nothing because they are only saved to the local copy, they are not seen by other calls to the mxm function.

See what happens when you instead declare the function like this, with an ampersand:

long long  mxm(int arr[],int i,int j,vector<vector<long long>> &t)

This makes it so that the vector is passed by reference, which is what you probably actually want.

Also, please consider not using GFG. It's an extremely poor-quality resource. The only thing they do well is the SEO that makes them sit at the top of Google results.

what is the time's differential between two submissions? Is it too distinctive?