When using a filter capable of removing over 99.9% of all microorganisms you must accept the fact that it will be more difficult to drink through.  The Frontier Max virus removal filter is just such a filter.  It will be more difficult to “suck” than a standard filter.  In the bottle one can squeeze the water through the filter quite easily.  In the hydration pack the pressurized reservoir greatly assists the flow.  The water must move through the filter at a slower pace in order to absorb the contaminants.  Any effort to speed this flow could adversely affect the virus removal.  For that reason it is important to have a “squeeze assist” system in place.