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Another workaround is to type your field reference in some plaintext editor and copy it directly to Confluence editor
ConfiForms is slower to render on Confluence 6.x than on Confluence 5
We have also noticed that - the same plugin code renders slower in Confluence 6 than in Confluence 5
We think, that this might be one of the reasons https://confluence.atlassian.com/confkb/slow-page-rendering-of-large-pages-due-to-http-post-limitations-183137.html
We have also noticed tht rendering times has been improved in Confluence 6.8.x, but is still is slower than on Confluence 5. We are continuing to investigate this issue and looking for solutions.