Default [Q] Do Android phones lag while downloading?

Do all Android phones lag when you download an app from playstore?

I've had several devices before, Xperia X8, SGS2, SGN, installed countless custom ROMs.
What I noticed that the UI starts to lag when I download an app from the playstore. After the install, the lag magically disappears...

Is that normal? I don't notice that on the iPhone, so I thought it might be Android-specifically.