I have a mixet shower/tub that has little water flow compared to the other bathroom. Changed the mixer still the same issue of low water flow

Are both hot and cold equally slow?

Make sure you have both handles adjusted so they give you good water flow. Also, some Mixet valves have "stop valves" on the rough body. Make sure both are fully open - and if they have filter screens - that those screens are clean of debris

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Milo