I had a similar problem with PCLOS ; Synaptic was taking forever to Refresh .(20 minutes ) I had a look at the failed connections and the ones which had connected and downloaded updates . There was only one ! What I found unhelpful was the instructions to enable "ALL" Repositories ! Dunno if that is what works best for most , but it just choked my ADSL connection . Errors such as ; "connection limit exceeded" gave me a clue . I unchecked all repositories , then selected the one site that had always connected and downloaded , plus the server closest to where I live . It has been lightening fast ever since.Thank goodness ! It was such a hassle initially , and pretty upsetting , I hate feeling like I'm sitting watching paint dry , or the lawn to grow
So I have to affirm the info pixellany gave as being similar to what I did , and greatly improved Synaptic performance.