Two changes to the screen where you pick which channels to add.

Sync now. RandomYV keeps a copy of your YouTube subscriptions rather than asking YouTube every time you open the page, which is what makes that page load quickly. The cost is that a channel you subscribed to five minutes ago is not there yet. There is now a Sync now button that discards the copy and fetches your subscriptions again, so a channel you just subscribed to shows up straight away.

You should not need it often. It is there for the specific moment where you went to YouTube, subscribed to something, came back, and it was missing.

The list loads as you scroll. The subscriptions list used to be paginated — click through pages to find a channel. It now loads more as you reach the bottom. If you follow a couple of hundred channels, scrolling is a great deal less work than paging.