have a Mac mini late 2012 with Samsung pro 512Gb SSD make clean intall to high sierra and the ssd now is APFS format supposed to be for better performance on SSD but now my mac mini turn on 43 seconds and before with sierra was 17 seconds and issues also with bootcamp windows 10 very slow... I make again clean install and back to sierra now is back again with super speed with extended jouneral format....
Same for a friend when he updated his iMac to High Sierra.... not sure what model but more recent than 2012. And have heard of others having a similar experience. However, given a bit of time, once the system settles and gets files all sorted, full functionality returns.
I had been reluctant to update my 2009 Mac Mini (5 GB RAM & original HDD) from Mountain Lion to El Capitan (not supported for MacOS 10.12 or later), given the suggestion by MacRumors x-perts & cynics that, without more RAM and a SSD, performance would suffer. However, I was starting to have some issues with some apps including MacRumors not working well without a more recent version of Safari, it became apparent that an update was increasingly necessary.......
...... So I bit the bullet a couple of weeks ago and did the update to El Capitan, which was relatively straight forward. Once installed it was a bit laggy initially, more so with some apps than others. Photos, for example, was initially all but unusable. However, given a few days it came right..... In most respects El Capitan has been a improvement. The advice of forum dilettantes was tosh.
Even Photos works OK now, albeit lacking some of the editing tools that I enjoyed with iPhoto. It seems they have been restored and more have been added and are available with more recent versions of MacOS..... Incentive to replace my nearly 9 year-old Mini at some stage, but not a high priority for now.
