There are about 8000 Mastodon instances and about 18000 daily active Nostr clients at the moment. So there will be no big impact if a Nostr account with reach posts a note with a link. Generating the link preview on the client is worse, because they are many more then relays. It is just a matter of numbers if this will be a problem.

F.x. the Telegram messenger generates and caches link previews on their servers. They cannot let the clients do this, because they have channels with hundreds of thousands of participants. They even have a bot (Webpagebot) to update link previews.

X does generate link previews on their servers and other services do that also. The problem with Mastodon is that there are so many instances and every single instance federating a toot will generate the preview again.

Web-Servers have ways to mitigate the problem also. The first response to the resource may just return the open-graph tags in the head and a redirect to the real resource or code to populate the web page that is not executed while generating link previews. That preserves the server from generating and returning the full content.

If Nostr grows significantly generating link previews on the clients will become a problem. I'm sure there will be a addition to the protocol to add and/or cache link previews for a note and avoid doing that on every single client again and again.