UITableView cell accessoryView image issue

The cells get reused. You need to reset the accessoryView each time. Its just a small change:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"TopicCell"];

    GPBTopic *topic = [self.topics.list objectAtIndex:indexPath.row];
    cell.textLabel.text= topic.name;

    if ((indexPath.row == 5) || (indexPath.row == 9))
    {
      cell.accessoryView = [[ UIImageView alloc ] initWithImage:[UIImage imageNamed:@"lock_icon.png"]];
      [cell.accessoryView setFrame:CGRectMake(0, 0, 24, 24)];
    } else {
      cell.accessoryView = nil; 
    }

    return cell;
}

Just for completion, you can also use Eric's solution like this - it may be faster, since the imageView is not created every time. But thats only a minimal difference. Probably your lags have other reasons.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = nil;
    if(indexPath.row == 5 || indexPath.row == 9) {
        cell = [tableView dequeueReusableCellWithIdentifier:@"TopicCellWithImage"];
        cell.accessoryView = [[ UIImageView alloc ] initWithImage:[UIImage imageNamed:@"lock_icon.png"]];;
        [cell.accessoryView setFrame:CGRectMake(0, 0, 24, 24)];
    } else {
        cell = [tableView dequeueReusableCellWithIdentifier:@"TopicCell"];
    }

    GPBTopic *topic = [self.topics.list objectAtIndex:indexPath.row];
    cell.textLabel.text= topic.name;

    return cell;
}

To showing custom accessoryView cell in Swift 4 & Swift 5

let lockIcon = UIImage(named: "lock_icon")
let lockIconView = UIImageView(image: lockIcon)
lockIconView.frame = CGRect(x: 0, y: 0, width: 24, height: 24)
cell.accessoryView = lockIconView