another not so fast removeObject

gistfile1.m

 NSMutableArray* ma = [[NSMutableArray alloc] init];
    
 // about 0.015 seconds
 for (int i = 0; i<10000; i++) {
    [ma addObject:[NSString stringWithFormat:@"%@ %d", @"ABC", i]];
 }
 // still slow: takes more than 7 seconds.....
 __block NSMutableArray* ma2 = [ma mutableCopy];
 [ma enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
     [ma2 removeObject:obj];
 }];