Late 2009 imac - which operating system update before it starts to slow down?

[EdBarnacle]

Hi guys, apologies if this info is available elsewhere, I couldn't find a direct answer.

I'm donating an old 'late 2009' Core 2 Duo iMac (12gb RAM) to a relative's kid and need to do a clean install for him. It hasn't been updated or used in a few years and so a new OS seems appropriate but I don't want to slow it down any more than necessary. Is there a sweet spot for OS versions that would work with it?

Thanks!
Ed.

 

[Fishrrman]

What OS is on it NOW?

For an older iMac, I'd suggest either 10.8 (Mountain Lion), which runs ok (Safari may give problems with some websites), or
10.11 (El Capitan) -- still supported by Apple, browsers work fine.

 

[elf69]

I have el cap on my 2007 imac but going to put sierra on it soon.

native, el cap on your 2009 but I ran the tool to put high sierra on my grand niece 2009 imac and it runs fine for web and word, office type use.

she has 6GB ram (max for her system)
I will add SSD when she can afford it, odd beachball now and then but that's down to the HDD.