Inserters carrying more than one item can only drop one item per tick. If the belts move too far per tick, then stack inserters, and normal inserters after stack size bonuses will likely be unable to fully compress fast belts on their own. If you load a base with construction chains built that compress at 60 UPS, and you load them up at 30 UPS, just check to make sure that you are getting proper compression where you need it.