First results my new version of DHT library @4 MHz
dht22_test.ino
4000000
LIBRARY VERSION: 0.1.15
Type, status, Humidity (%), Temperature (C) Time (us)
DHT22, 21 Checksum error, 67.7, 21.2, 5600
DHT22, 14 OK, 66.1, 21.1, 5648
DHT22, 27 OK, 66.0, 21.1, 5616
DHT22, 26 OK, 65.9, 21.2, 5600
DHT22, 14 OK, 65.8, 21.2, 5536
DHT22, 14 OK, 65.6, 21.2, 5504
DHT22, 14 Time out error, -999.0, -999.0, 6880
DHT22, 27 OK, 65.4, 21.3, 5664
DHT22, 14 OK, 65.1, 21.3, 5600
DHT22, 15 OK, 64.9, 21.3, 5504
DHT22, 14 OK, 64.7, 21.3, 5696
DHT22, 26 OK, 64.6, 21.3, 5648
DHT22, 7 OK, 64.5, 21.3, 5520
DHT22, 26 OK, 64.4, 21.3, 5600
DHT22, 14 OK, 64.3, 21.3, 5616
DHT22, 14 OK, 64.3, 21.3, 5600
DHT22, 14 Checksum error, 115.5, 21.3, 5616
DHT22, 14 OK, 64.3, 21.3, 5600
DHT22, 14 OK, 64.3, 21.3, 5600
DHT22, 26 OK, 64.4, 21.3, 5600
TOT OK CRC TO UNK
20 17 2 1 0
....
DHT22, 19 OK, 66.2, 21.1, 5696
DHT22, 26 OK, 66.2, 21.1, 5696
DHT22, 14 Checksum error, 66.3, 225.9, 5712
TOT OK CRC TO UNK
100 82 12 6 0
....
TOT OK CRC TO UNK
200 166 25 9 0
Score of ~80% is not too bad