Kafka restart takes a long time

A 3 node kafka broker cluster went down yesterday (all nodes) and I just
noticed it this morning. When I restarted it this morning, I see a lengthy
list of messages like this:

Loading log 'mytopic-partitionNum"
Recovering unflushed segment 'some number' of in log mytopic-partitionNum.
Completed load of log mytopic-partitionNum with log end offset someOffset

It's been going on for more than 30 minutes since I restarted the broker. I
have quite a few partitions (over 1000) but I still wouldn't expect it to
take such a long time.

Any ideas on how I should investigate the problem?

Thanks!