I do have since 15-18 month a Mac Air Book, 4 GB RAM i5 processor.
Since couple of weeks the perfomance is reducing, cooler is to "active". I do assume due to some small vulnaribilities.
I have over 15 years experence in supporting IT and IT Infrastructure and over 4 years giving support on Mac Systems. I personally own a Macbook Pro and various other Apple Devices so I am pretty sure about any problems related to Macs and IOS Devices