I posted this in another thread, but it fits your post exactly:
Anyone that complains about system performance that is still running a spinning platter has no leg to stand on.
Moving forward, you simply need an SSD for acceptable speed, (read: MacBook Airs, rMBP, Fusion Drive for iMac, etc.)
Also, many, many apps require Mavericks for the new API's (just check the App Store: Pixelmator, Omnifocus 2, Final Cut, Sunrise Calendar, OneNote, etc.).
For any actions that you can still use SL for, there will be more in the future that begin to require Mavs. Just ask any developer.
Furthermore, you can expect to need to upgrade your hardware just the same for Yosemite performance, so you might as well do it now. You're only delaying the inevitable.