Yosemite Safe to upgrade?

Question

I absolutely love Apple's new OS ( Yosemite ) for its Computers ( iMac, Macbook Pro and Air ), but I am a little hesitant on installing it when it arrives.
as one of my friend has an iMac ( Mid 2011- 12,1 ) and he says that after installing OSX Maverics ( Clean Install ) his iMac has become significantly slow in tasks simple tasks!
So i am really worried that if i Upgrade to OSX yosemite will it slow down my iMac?
Info about my iMac :
iMac ( 21.5 Inch, late 2013 )
8GB 1600Mhz DDR3 RAM
Nvidia GeForce 750M Graphics Processor ( 1024MB )
1TB 5400RPM HDD.
I wanted to have 16GB of RAM and 1TB of FusionDrive but in my country India Apple doesn't permit the end user to upgrade the iMac even while purchasing it!
they have preset a list of iMac's with different configurations and we have to buy those and they cannot be upgraded in any way...the RAM was upgradable in iMac's till 2011 but from 2012 Apple made the RAM non user upgradable in 21.5 Inch iMac's! so cannot upgrade that as well on my own.
PLEASE help, Thank YOU in advance.
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Answer 1

I have a MacBook Pro 8,2, which is a few years older than your iMac, with 8 GB of RAM. I put a 500 GB SSD in it. It is hard to predict how your Mac is going to perform with Yosemite without knowing exactly what you are planning on doing with it. I do a fair amount of audio and video recording and editing with my MBP, and it works very well, fast and reliable. It worked very well with Mavericks. (we are not supposed to discuss the Yosemite Βeta on this forum but that works very well too.)
Your friend's experience with Mavericks is probably not from Mavericks per se. The vast majority of the problems that people report with Mavericks performance on this discussion are due to user-specific issues, typically "crapware", like "cleaners", or "optimizers", (e.g., CleanMyMac or MacKeeper) that they shouldn't have installed in the first place, or third-party antivirus software, which is of dubious utility on Macs. Or a hard drive that is too full, or trying to run fourteen apps at startup with 2 GB of RAM, or having 57 incompatible launch daemons running. Etc. Etc.
I would suspect that your Mac would probably work pretty well for routine, non-memory or processor intensive tasks, like word processing or checking your email, but you might not be happy with its performance if you are editing audio or video or using Photoshop, considering your HD, which is relatively slow (5400 RPM). But that is a guess. Maybe someone with a similar iMac with your configuration can comment. If you can upgrade your HD to a SSD then your iMac would probably work very well. Very simple to swap out the HD on my MBP if you have the correct screwdriver bits; I've not tried it with an iMac.
Your HD is certainly big enough that you could create a separate test partition just for Yosemite and try it out. If you back up your current installation with Time Machine or Carbon Copy Cloner first, even better, and it would be simple to go back to your current installation if you don't like Yosemite.