Late 2013 iMac slow to boot to desktop

The startup process has several stages. Please see this support article and determine at which stage the delay happens. How long is the delay?

Hi , I bought the same late 2013 iMac with the same config like yours and I experienced its slow performance from day 1 and it cannot be resolved simply because my iMac is using a 1TB 5,400RPM conventional Hard Drive. Unfortunately it can't be replaced as the 21" model is manufactured in fixed config, unless you ask Apple to replace it with a 1TB Fusion drive or 128/256 SSD drive.

So I went off to buy an external 240GB SSD drive and I installed Marvericks in that SSD drive housed in a USB 3.0 case and connect it to the iMac as an external boot drive, and after that, my iMac is re-born with perfect performance, thanks to the fast speed of the SSD and the USB 3.0 connection.

You can consider doing this using even a lesser capacity SSD, like a 120GB SSD, which in Hong Kong it only cost HK$600 or less, which is US$75 or less.

This is a best option without going to Apple for help, and you can do it yourself

Hope it helps. And for any question, you can contact me at lks3197@gmail.com or lks31970602@me.com

Up on Yosemite here. Make sure you don't get caught in the simple trap I got hooked into. I installed a new SSD in an open bay in my 2011 iMac. Worked fine for 6 days, screaming to the desktop. Then on the 7th day it started taking about 1 min and 50 seconds for the Apple logo to appear, and only then did it resume screaming to the desktop in about 15 more seconds. I did all the usual - i.e. PRAM reset and checking the Sys Prefs Startup Disk selection etc. Also did repair of disk permissions - twice. Finally I remembered a tech once told me to pull all cables from the Mac, including the power cable for 30 seconds, then plug it back in with nothing else connected. Bingo - screaming start again. Shut down reconnected all cables and still OK. But one more thing, before I shut down and restarted, while I had a hard wire network connection, I noticed that the Wifi was turned on and hunting. Think that may have been a contributor. I noticed in my initial Yosemite configuration on the SSD I was asked which network I wanted to join. The only two options as I recall were my two Wifi connections - i.e. main router and hot spot, and unthinking I selected one of those, forgetting that I didn't need it. Not sure if the setup would have taken if I had omitted selecting one of those wifi connections or not. In any event I can't explain why it worked fine for 6 days and then, after an overnight shutdown, suddenly started being sluggisth - but bottom line here - be sure to pull all cables for 30 seconds before restarting.