That is not possible (breaks Modbus rules) and I don’t know what you’d get instead. Either allow to default (35mS) or set something realistic.
By default, binding TCP mode opens/closes TCP connection at each poll, which will come with an overhead.
More detail here -
Please be aware if you edit Thing parameters you may need to restart the binding package (or openHAB) to get the new values used cleanly.
Please use DEBUG as shown in the binding docs, choose which connection method you want to investigate, and show us some logs so we can see where the time goes.