This sounds like your first time "Boosting" a toon through Deadmines or one of your first times at the very least. First off like Skuggo pointed out your toons do not need to be very close at all to receive xp. You can get almost all the way to the first boss before they become out of range to receive xp infact in DM. This will result in much much fewer deaths than say if you have your alts on follow 2nd as Skuggo also pointed out you will get better and better with each run in effiency. You'll pull better, faster, and with fewer deaths the more practice you get. I used my paladin to boost instead of my mage so I'm not use to mage times but my paladin can generally clear, run back, reset, and begin pulling, in 15 mins. on avg. Again however I've had much practice.

Also wouldn't recommend bothering to loot in Deadmines. It takes way too long and its not even close to being good money imho. You can masterloot the blues to your alts if you want but if you're planning on pwrlvling them as far as you can you'll probably not even need to bother gearing them till you hit ZF maybe SM.

Have some patience and you'll be busting out lvls and outta DM in no time.

The seeming normal boosting schedule is as follows...

1-15 questing
15-20 DM
20-25 Stocks
25-40 SM
40-45 ZF

After that it varies quite a bit some ppl go to ST and then strat, some just quest. I personally stayed in ZF till 50.