Quite weird, Njump expose standard opengraphs and I never had any problem with preview in Telegram and other clients. I just did a test (Damus on macOS) and it worked fine.
If you open the links the is the page load immediate?
The only additional payload is the text-to-image preview (that occuours only in some situations) that is generated the first time a note is shared. But should be a sub 300ms response.
On another your note in this discussion i read:
> iOS makes the previews not me. Its apples standard preview thing.
So maybe IOS has a really low timeout and it is also a geo-location iusse, as bu5hm4nn (npub1hlq…z5wg) pointed out?
Btw, njump.me server is in Europe.