Is there a way to disable showing what your friends are playing when first logging into steam?

There's not a "only after login" option for displaying these friend notifications. As has been noted, it's all or nothing in Steam's settings. There's also no launch option that helps.

However, you can make it go faster, if you edit your Steam style. In your Steam directory, look for resource\styles\steam.style. (Assuming you use the default skin - if not, check the \skins\(skin name)\resource\styles directory) Back up this file, and then you can edit it. Search for "Notifications." and you should see a block like:

Notifications.PanelPosition     "BottomRight"   // Can be "BottomRight", "BottomLeft", "TopRight", "TopLeft"
Notifications.PanelPosition     "TopRight" [$OSX]  // the dock is on the bottom for OSX, so instead pop top right like other apps do
Notifications.SlideDirection    "Vertical"    // Can be "Vertical", "Horizontal", "None" and controls slide effect
Notifications.FadeInTime      "0.45"    // Controls the time it takes to slide/fade into view
Notifications.FadeOutTime     "0.45"    // Controls the time it takes to slide/fade out of view
Notifications.DisplayTime     "6.0"     // Controls the length of time at steady state after fade in and before fade out
Notifications.StackSize       "3"     // Controls how many panels we will stack before background queuing

You can move the notifications to some other corner, have more on the screen at once, and make them cycle faster if you so desire.

The only way to disable this is to disable friend notifications entirely.

In Steam's settings, go to the Friends tab and uncheck "When any Friend joins a game" "Display a notification".

Edit: As @ardaozkal has pointed out, you can override this per friend by opening the friends list, clicking the down arrow next to their name and clicking Notification Options. Then, under "When [person name] joins a game" check the "Display a notification" box.