The bars on your phone's onscreen indicator are a rough indicator to the quality of the received signal. The speed you get is dictated by the capacity/spectrum available on that site - you can get poor speeds over a strong signal as well as excellent speeds over a poor signal.
That said, is this a recent problem? There may be a temporary network issue causing this if so. Check by using "check status" on https://coverage.ee.co.uk/coverage/ee
