It's pulling down all your gift wraps and decrypting them to determine if they are calendar events or something else (DM, etc)
Discussion
If you use lots of nostr private DMs there will be lots for it to run through.
This is a less than an ideal way to handle this, IMO. That is a LOT of stuff getting pulled down and it would not be friendly to those on slow connections (like I am) and on lower performance hardware. I am not sure that I have a good suggestion on how to solve this issue, as I don't know a heck of a lot about how all that stuff works. I would think that one could use a tag of some sort to designate an event "outside" the encrypted data so that you only pull down the messages that are tagged with the calendar data tag.
Yeah I went back and forth on how to handle private events quite a few times. I had debated doing another relay list kind similar to DMs list for users to define calendar relays but held off because it would still be using the same gift wrap standard so it wouldn't guarantee DMs / calendar gift wraps cross populate even if desired relays were defined. I'll definitely be keeping my ears out for any suggestions to get past that. I certainly agree it's not the most ideal setup for light-powered devices.