Old cellulose insulation holding heat during summer
UPDATE: This answer is too simplistic and lacks understanding of the true issue. See a superior, more complete answer here: Ridge vent conversion creating heat problem

The insulation is not holding heat (i.e. acting as a thermal mass). The likelier problem is that your attic itself is what's holding heat. Attic heat gain during the summer is primarily caused by sunshine. The sun hits your shingles and heats them up, and that heat radiates down into the attic. The darker the shingles, the more pronounced the effect.

As a result, an attic during the summer can easily reach temperatures 20-40 degrees higher than the ambient temperature due to this solar gain, even in your relatively cloudy climate. You can see for yourself by going into the attic around noon or 2 PM. It should be punishingly hot.

It's your insulation's job to prevent all that heat from getting through the ceiling and heating up the house. 6-8 inches of cellulose (R-3.5/inch) + 1 inch of rock wool (R-4/inch) works out to R-25 - R-32, which are numbers that are decent but not great for an attic. You need more! Blow more cellulose in there; 10 or 12 inches is a good start.

Another approach involves changing the color or material of your roof. The next time you have to redo it, if you choose light-colored shingles, or even light-colored metal, that will help tremendously to reflect the sun's rays, preventing a lot of that heat from even making it into the attic to begin with. But by the same token, this will hurt you in the winter, when that heat is actually helping to keep your attic warmer than the outside temperature. The degree of the penalty depends on how much of a heating load you have compared to your cooling load. Adding more insulation will help with both and is the recommended approach.