https://www.esp8266.com/viewtopic.php?p=61322#p61759
many boards like NodeMCU or WemoS D1 have input resistor diver chains to allow 0-5 inputs and effectively divide that input range down to 0-1volt for the ESP, in which case the input impedance of these boards would be much lower. The Wemos D1 mini for example has a 220K and 100K series divider giving you an input impedance of < 100Kohm.
I think that means you should be using resistors well below 100k in your voltage divider. Try 10k and 2.2k.
Or, if you want a lower passive current draw when your Arduino is turned off, add an op-amp voltage follower. That will turn a high impedance voltage source into a low impedance voltage source.