This could be a number of things, from the service being provided by your line, to your router or general set-up being slightly wrong.
You appear to say that you have done the 'usual' clean-up routines, and very little improved. So what I would suggest, is that you check out your ISP support website for further information. If your router was not supplied by your ISP, then check the website of the router manufacturer. Google might also be your friend.
Further to the above, a reset of the router, with speed-tests, before and after, might find something.