Time Machine is EXTREMELY SLOW. What is the best way to make a complete, reliable copy of the hard drive before i erase the disk?

I was given an older iMac (from 2010) on the condition i first create a complete, reliable copy of everything on her hard drive and then wipe it clean. Is Time Machine the best way to do this so that she can use this data when she needs it? I started Time Machine but it is so slow, at this rate it will take at least 10 days to do the first backup. I'm already on the 3rd day now at about 2.5 GB per day. Is there a better way that is reliable? perhaps some kind of disc copy? This iMac has 4gb memory,processor: 3.06 gHz Intel core i3, model iMac11,2, 1 processor, 2 cores. thank you