Unity's faster implementations for string operations found: https://docs.unity3d.com/Manual/BestPracticeUnderstandingPerformanceInUnity5.html

Unity Custom String.cs

public static bool CustomEndsWith(string a, string b) {
        int ap = a.Length - 1;
        int bp = b.Length - 1;

        while (ap >= 0 && bp >= 0 && a [ap] == b [bp]) {
                ap--;
                bp--;
        }
        return (bp < 0 && a.Length >= b.Length) || (ap < 0 && b.Length >= a.Length);
}

public static bool CustomStartsWith(string a, string b) {
        int aLen = a.Length;
        int bLen = b.Length;
        int ap = 0; int bp = 0;

        while (ap < aLen && bp < bLen && a [ap] == b [bp]) {
                ap++;
                bp++;
        }

        return (bp == bLen && aLen >= bLen) || (ap == aLen && bLen >= aLen);
}