I understand that it is in the works of being implemented, however I cannot emphasize it enough desktop client support, specifically outlook (I know, I know). When dealing with emails and calendars, with any business, things can complex in a hurry. I run a networking/surveillance installation business, and I am soon going to start into real estate. So in a sense my first real suggestion would be to add more complex repeat options to match other mainstream calendars. I did run into issues however with one of my calendars not importing all events resulting in an error because of the complexity of repeat events, however I was able to find these event specifically and bypass the complexity portion. However with their import tool, it was incredibly painless. ics calendars was a pleasant surprise! Before looking into proton a couple years ago, I was well involved in the apple infrastructure, and up and just switching calendars was a no go because of previous events, and it would take several hours if not days to successfully manually migrate everything. The new ability for ProtonCalendar to import.

The only exception being when initially opening the calendar or switching apps, which is understandable because of encryption handling. This is a large improvement over almost any other calendar, especially gmail and the outlook web gui. My overall feel of the calendar and how snappy it is, is amazing! Every click has an instant reaction, including going into settings and making any changes. My opinions and suggestions are only my own from my own experience, so please be nice and respect mine, as I will respect yours as well :) I joined the protonmail/calendar/contacts beta early this week and have used it consistently throughout the week to see how it works.

Protonmail calendar