By default Nanominer sets the memTweak option to 1. This slightly improves memory timings. However it introduces a little bit of UI lag. This can be a problem if you are trying to use your computer for other tasks. The simple resolution is to change/add the memTweak setting to 0.
wallet = WALLET_ADDRESS ; Coin to mine. coin = ETH ; (Optional) Rig (worker) name. rigName = EthMiner memTweak=0