I have just gotten around to updating my early 2009 Mac Mini (base 2 Ghz model with an extra 4 GB of RAM, for a total of 5 GB) from Mountain Lion to El Capitan. I stuck with Mountain Lion so long because it was fine for my needs; I had no need of the connectivity and other features offered by more recent versions. However, some websites were no longer working well with Safari (including MacRumors!), and Opera (which I have used with them) had an update and no longer worked with OS X 8.5. I'm reasonably pleased with how most apps work, but not Photos. With Mountain Lion, iPhoto worked well, and was fine for my needs. I use a Fujifilm X20 to take shots of sports for websites and Facebook pages, as well as some for the local press. The 12mp files are not too demanding, and there was no perceptible lag when editing them. Different story with El Capitan and Photos. The lagginess is quite marked, and makes editing difficult to say the least. Is there anything I can do to improve the situation?