Slow Responses Initially

My wife and I both have our individual accounts on 1 iMac.

When I switch to my account after my wife has logged into hers, my wallpaper picture initially covers only about two thirds of the screen. After about a minute, it expands to cover the full screen. Also, when I switch to my account, the internet is connected but is very slow for about 2 minutes. after this time it is fine.

Anyone got any ideas on how to speed up these 2, initially slow responses?

I am running Mavericks OSX 10.9.5.

Many thanks in anticipation.

Andy