When it cools, bridgmanite transforms into another mineral called post-perovskite, which is even more thermally conductive and would therefore increase the rate of heat loss from the core into the mantle.
"They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inactive much faster than expected.".
Mars is cooling a bit faster because it's significantly smaller than Earth, but there are other factors that may play a role in how rapidly the planetary interior cools.