I knew you would chime in with positive advice for keeping the old-girl running.
Great to hear your 2007 iMac is doing fine with El Capitan and 6gig or RAM!
Maybe the OP's hard drive is getting full, needs maintenance, or is starting to fail (leading to the slowness issues).
- Nick
Thanks Nick, as is my old 2007 15" MBPro 2.2 GHz 4GB still quite operational for most of our needs.
Both have had their HDDs replaced, WD 1TB Black in the iMac and a 240GB Crutial SSD in the MBPro.
Assuming their HDD has enough spare room, I always found a good maintenance speedup was to make or use a current backup CCC clone, boot from it and wipe and nuke the internal HDD with a zero-out or one pass security using Disk Utility, (that maps out any bad sectors), and a step that must not be skipped, then use CCC to clone it back.
The increased speed difference was always quite incredible for our's and others with their HDD Macs with the same procedure performed.
PS: Using SuperDuper! for some reason did not produce such dramatic results as CCC (Carbon Copy Cloner) would do.
Sure can't hurt to try.
- Patrick
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