Why does it sometimes need such a long time to log into the light wallet? Is it depending on how many transactions are already done inside the seed or is it depending on when was the last snapshot? Or is it a development problem that will be fixed in the future?
It has been observed that the more addresses in your wallet, the longer it takes for your computer to retrieve all associated addresses from the Tangle.
The new wallet should remedy this -- it will be stateful, and store your previous addresses locally, so you don't have to re-retrieve them every single time you log in.