I have a iMac (27-inch, Late 2012) 2.9GHz i5 I have 8GB memory from Apple (2x4GB) and I installed 16GB(2x8GB) crucial (CT2KIT102464BF160B) and from booting to using the computer it is super slow only even typing sometimes there is a huge lag.   I have installad on the following configuration from Top to Bottom slot when unit is upright    8GB - Crucial memory    4GB - Apple memory    8GB - Crucial memory    4GB - Apple memory   if I swtch positions my computer won't turn on just beeps. ... View more