it could be that your pipe is heavily coated with limescale. try unclogging your drain as normal, rinsing REALLY well, then using limescale remover to see if this helps. If it helps but only a little bit, sadly this could be an indicator that you have heavy scaling and may need the pipe replacing. The upside is that it could cure the problem or cure it over two or three treatments if the scale is not too bad
With any luck it may just be a build-up of soap deposits and hair choking the outlet pipe. Any easily available drain-cleaning product should clear it but be careful if you use caustic soda because of the violence of its reaction with water.