yes it does reduce the efficiency if you squash it down into the ceiling joist zone and board over it.

you can buy much denser thin insulation products in polysterene such as Kingspan and Celotex. kingspan isnt too pricey, but celotex is quite expensive but has amazing thermal properties.

or some people batten above the existing ceiling joists to be able to put more insulation in, but this is adding more work.

you will get condensation with ANY type of loft insulation if the roof isnt ventilated at eaves and ridge. i have a new house with 10 inches of fibreglass insulation quilt and within a few months the loft was full of condensation because the builder hadnt put ridge vents in.

once they put the vents in, its been fine ever since.