...... CPU is 100%. And it is no longer smooth as soon as filters are added.
Are you initiating preview rendering after the filters are added? Also, double check your Audio Hardware preferences. The Default Input should be "None". Setting to none may relieve some of the CPU workload as it plays previews.
There are also settings on the General Preferences page that may make a difference. The Hardware Acceleration setting may have a a backwards effect. Turning it off has been know to improve playback. Try the different settings under Timeline Render Quality too.