Hello,
Just to add to what vpradeep01 mentioned from that book:
This Log data is not flushed to capacity-tier on host reboot and thus the Log recovery time (and thus reboot time) correlates with the amount of Log data on the cache-device - my understanding of this is that this data also needs to be compared to the current state of the data as data will change on the rest of the cluster while the node is absent and thus if you have a high rate of change on the cluster then this will add to the time needed in SSD initialization (as will unsupported things such as increasing the default assigned high and low Log-capacity levels).
Bob