Will RAM help?

[[ Any thoughts on whether I'd be better off upgrading to 8gb ram (from 4gb) or instead upgrading to a SSD? Or something else? ]]

Replacing the platter-based HDD with an SSD will make the biggest difference.
Far more than adding RAM.

But -- you should do both.

I take it that Yosemite (like Mavericks before it) was developed on Mac platforms that had both plenty of RAM -and- SSDs. Yes, it can run on older HDD's -- but, the "running" will be more like walking insofar as the "user experience" goes.

I still keep my 2010 MBPro on 10.6.8.
I replaced the HDD with an SSD, and it boots in 5 seconds. Runs like lightning!

 

Agree completely. Adding and SSD will be like night and day. And 8GB of RAM is cheap, so do add that also.

 

SSD definitely. After adding memory you will only see difference when opening many apps, opening big apps (Photoshop), or play with fast user switching. 4GB is rather min for OSX, so upgrading to 8GB as quite cheap and quick method should be also done

 

Mavericks for whatever reason is very slow if you have an HDD, and I'd assume it's the same with Yosemite. My Mac Pro has 6GB of RAM but has been fine with Mavericks ever since I put an SSD in.

I'd go for the SSD, not only because it'll probably speed up your Mac a lot but because it'll make it more reliable since HDDs are subject to damage by movement. Having more than 4GB of RAM is usually overkill anyway. I never use that much unless I've got VMs running.

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This could also be it, but I experienced such problems in Mavericks (apps open slowly, minor keyboard delay) even with a good HDD that runs Mountain Lion fine.

 

Could it not be that Yosemite is simply still full of bugs? Perhaps wait for the next few releases.

But if it's really your hard drive, you should better do a backup soon. Good that you mentioned that Weaselboy, because I believe I have a computer with a similar symptom!

 

I've seen a number of posts in SSD forums (and here on macrumors) where a new SATA cable helped on a Unibody MBP, so it's reasonably common. I'd go for an SSD, but be prepared to replace the SATA cable too.

 

Weselboys answer is the only 100% correct one.

Avoid weird memory monitoring/cleaning apps. Use only the activity monitor.