How below code have two different complexities?

I am solving one question of leetcode and while solving I have written one solution which is comparing if a part of string is palindrome or not. But when comparing with customized code of Time Complexity O(N*N) then it exceeds the given time limit (TLE).

But when I check equality of two strings after creating substrings of them then it does not give TLE even if the time complexity is O(N*N), N for outer loop and N for creating substring and comparing them.

Question:

214. Shortest Palindrome

You are given a string s. You can convert s to a palindrome by adding characters in front of it.

Return the shortest palindrome you can find by performing this
transformation.

Example 1:

Input: s = “aacecaaa”

Output: “aaacecaaa”

Example 2:

Input: s = “abcd”

Output: “dcbabcd”

Solution that exceeds the time limit:

var shortestPalindrome = function(s) {
   const n = s.length;
   const rev = s.split("").toReversed().join("");

   let f = (i) => {
       for(let j = 0; j < n-i; j++) {
           if(rev[i+j] != s[j]) {
               return false;
           }
       }

       return true;
   }

   for(let i = 0; i < n; i++) { \ this looping for n times
       if(f(i)) { \ roughly taking n time for comparison
           return rev.substr(0, i) + s;
       }
   }

   return "";
};

Working solution:

var shortestPalindrome = function(s) {
   const n = s.length;
   const rev = s.split("").toReversed().join("");

   for(let i = 0; i < n; i++) { \ this looping for n times
       if(rev.substr(i) == s.substr(0, n-i)) { \ this line must be taking O(N) right?
           return rev.substr(0, i) + s;
       }
   }

   return "";
};

Please explain how these two solutions are different.