Is custom token module sustainable for masive use?
In general - no, this shouldn't be a performance issue for your use case. The main problem identified with token module and pathauto is in generating the Token UI screen when dealing with a large number of tokens. There are good discussions on this here on stackexchange but I've also found that using the token tweaks module can resolve most of these.

In your use case case, your using just one token to generate the URL path which is then saved to the path alias table, a table optimized and indexed for performance. The only real demand is on token implementation but this is only done once, when saving the article and you're using objects that are already loaded (ex. $user, $node->nid) to generate it so it won't be a performance issue.