My water pressure dropped like to 65% of what it w...

My water pressure dropped like to 65% of what it was before their system was installed. I specifically asked Robert from their sales/service team if this would happen and he assured me we can a max drop of 10%. But this is not true. Another neigbor of mine is having the exact same issue.
Two people cannot even take showers at the same time or run your dishwasher or washing machine at the same time. I have changed the filter twice in less than 4 months and its still the same result.
Also the master certified plumber who installed the system that the parts being supplied with this system are sub standard and have sprung leaks for at least 2 other customers of his.