1) consider upgrading to 8 gigs.
yosemite likes to use every drop of ram to make your mac faster. and it can be a little sluggish on a 4 gig machine
2) reset pram and smc
3) did u turn on file vault? file vault can slow down the mac like 30 %
4) be patient. macs make application caches, and once it has enough application caches it will get better
5) run disk utility to verify and repair the disk and permissions
6) maybe get a hybrid hard disk. they come with 8 gb ssd or more + a mechanical drive
7) uncheck the resume from last lesson box once in a while to clear that out
8) download applejack from source forge
boot into safe mode by doing command S
and type applejack AUTO
the utility is a little old but it still does a good job on mavericks and yosemite
theres also a deep clean option
i think replacing it with a SSD should be the last thing to do.
i got a new 2014 mac mini with 4 gigs of ram with yosemite. when it was brand new it was sluggish as heck and
i wanted to return it. but after having it for a week or so , it has gotten a lot better.
people forget that os x makes application caches. and it needs time to build them
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post a screen shot of activity monitor. with cpu and memory
as long as MEMORY PRESSURE is LOW and GREEN you are perfectly fine and you don't need a ram upgrade.
if its using a lot of swap and compressed ram, you need more memory
if it says memory 4 gig and 3 gig used or 8 gigs and 7 gigs used or something like that. that doesn't mean you are running out of memory. that just means Yosemite is working properly to make your system snappier.
the genius told me that , and it bugged me
i felt like driving back and complaining to the store manager