SUMMARY

Currently it is possible to subscribe to a remote (http) ical calendar file by adding a new iCal-calendar-file calendar. But this current UI is a bit problematic:
First the discoverability of this way is quite bad: The description talks about adding a *local* ical file, while a user wants to add a remote ical file in this case. Then, the field to paste the calendar URL into is labeled "File Name" and has a folder symbol to its right, again implying only local calendar files. I even suspect that the possibility of pasting an http URL into the "file name" filed is just a coincidence of KIO handling the http magic in the background.

Second, several optimization options are missing from that dialog field: Refresh interval is very important for remote calendar files, as they cannot be watched using inotify and also too frequent refreshes of large ical files might lead to a high load.
I suspect that a manual refresh interval (e.g. 30min) might help with high-load issues like #355521

So I suggest adding a new dedicated calendar type for remote calendar URLs, supporting all common formats (ical, webcal) in the same UI and providing additional options specialised for remote calendars (refresh interval, …)