Node should throttle repeating balance reads and respond with cached data.

[bartossh]

Repeating balance reads will significantly decrease the performance. The read process requires the accountant to traverse the DAG from the fresh leaf to the current genesis.
To avoid DDOS attacks there is a good reason not to respond to recurring requests, and or even blacklist the wallet.
It will be a good idea as well to cache the balance read after the first read and remove the cache value after token transfer of given wallet.