Booting takes a long time

This can mean one of two things:

1.) You have a very full hard drive and the computer doesn't have enough space to run virtual memory. On a fast computer there is normally anywhere from 80 GB to 200 GB of space on a hard drive that can be used as virtual memory. Don't know what virtual memory is:

http://en.wikipedia.org/wiki/Virtual_mem...

To fix this you could upgrade the hard drive to a larger on e of clean out your system.

2.) You have a failing hard drive that my have trouble reading data, therefor causing the system to run very slowly and for it to temporarily lock up.

If you have this problem then you will need to get a new hard drive and a 3.5" SATA hard drive reader so you can copy your files from one hard drive to the next. I would recommend using Carbon Copy Cloner to transfer the files. I would also use this guide to replace the hard drive:

iMac Intel 21.5" EMC 2308 硬盘更换

If you want any clarification feel free to ask and I will get back to you promptly.

Hope this helps.

Also try disconnecting your USB devices other than the keyboard or mouse if they are USB connected and reboot. If you have a bad USB device or cable the system will also behave like this as well.