Bitcoind uses only ~1.5% CPU for RPC command "getnewaddress"
This is a known problem in the wallet code.

Whenever a new address is retrieved from the key pool, a new key is also generated to replenish the key pool. When this happens, we try to make sure this new key is safely on disk, so we flush the wallet file to disk. Due to probably overly aggressive flushing, this results in large writes to disk in BDB.