My 2007 iMac runs slow and occasionally freezes on startup

Question

Hello,
iMac overview:
iMac (20-inch, Mid 2007)
Processor:  GHz Intel Core 2 Duo
Memory: 3 GB 667 MHz DDR2 SDRAM
Graphics: ATI Radeon HD 2400 XT 128 MB
OS: OS X Yosemite, 10.10.2
250 GB SATA Disk shows 154.58 GB free of 249.2 GB
I have been having the following issues with my iMac since upgrading to Mavericks, and they've persisted through to Yosemite...
Issues:
intermittent "data crunching" sound that will last for up to 10 minutes, causing active applications to stall
if I drag the cursor over the dock early during startup, the computer will freeze
if I launch an application early during startup, the computer will freeze
Mail occasionally "re-counts" and reloads the messages in my inboxes, causing Mail to stall or freeze
Any ideas? (If it's recommended that I provide more information gathered from Terminal, I'll need clear instructions.)
Thank you

Answer 1

Slow performance after installing Yosemite on an older Mac is often caused by inadequate memory. Although you can install the upgrade on a Mac with 2 GB of memory, experience has shown that at least 4 GB is needed for full performance.
Your model has upgradable memory. Install as much as it can take, or at least that much. Any reputable RAM vendor will have a web form in which you can select the Mac model and be directed to compatible parts.