slow connecting to some sites on some computers

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My wife's OS X 10.7 iMac starts to download a site with safari. The blue pr
ogress line move a little then stalls. I have no problems on my OS X 10.10  
MacBook Pro. This has been happening for about two weeks. She often has man
y apps open at the same time but the iMac has 12GB of memory. She does not  
have many safari windows open at the same time. She gets similar results wi
th Firefox.

Both are using wifi. The router is a D-Link DI-624. The router is closer to
 the MBP than the iMac.  

Any help much appreciated.

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