I realise that.
I'm asking because whenever I start my iTunes, I get that icon in the top right (as shown in the screenshot) and my iTunes is frozen (and useless) and I thought that this might be what's happening to you too.
On the odd occasion that I've been quick enough, I've managed to click on View / Show Activity Window before iTunes actually freezes and in the pop-up window that appears, it tells me that iTunes is "making my purchases available".
I don't think I'm being unreasonable in thinking that it shouldn't be necessary for iTunes to check this every time that I start it and to make itself totally unusable to the customer (that's me) until it's finished.
Since I haven't managed to find a way to prevent iTunes from indulging in this navel gazing I simply put up with it, but it's extremely annoying.