Originally the rss feed was nearly 9 seconds direct but would intermittently work on the forum (preview very rarely worked), the feed was then re-written using multi-cURL to speed it up.
Another similar feed (3 seconds direct, multi-cURL) works fine and previews in 3 seconds.
As a temporary measure, the rss feed is run on another server (about 1 second direct) and the xenforo preview happens in about 1.5 seconds.
There appears to be some interaction between different news feeds, when I deleted one that was not working on the forum, another one that had also not been working suddenly ran ok and posted on the forum.
Any suggestions where I should be looking? I initially thought it a server problem but the rest of the testing would suggest its xenforo.