You don't specify exactly in your question what is taking long to respond. Is it your internet access in general, or is it just your webmail/email access that is slow? If it is just the latter, then changing ISP is probably a bit drastic. Just get a free email account from the likes of Google which can be accessed using an email program like Outlook or Thunderbird if you so choose to.
If it's the internet speed in general then try downloading and using a program like Malwarebytes (free version)
http://www.malwarebytes.org/ and doing a full scan of your system to, firstly, rule out malware and viruses that may be slowing your system and net connection down. Bear in mind also that evenings, particularly between 4pm and 9pm are considered a peak time for national internet access, and networks are usually more congested at these times - although 5 minutes is taking the biscuit. I am assuming it is broadband access, as you haven't said? If so, what speed is it rated at?