I just had my talk page converted to Flow on Wikidata and I notice Flow is often very slow to load (also happens on this page). A dimmed preview will appear quickly, but it can then take over 14 seconds before you can actually interact with the page. All the time Firefox is indicating it is waiting for some stuff to be loaded from www.wikidata.org/upload.wikimedia.org. Occasionally the whole page will load in about a second, so it seems this is not inherent slowness, but that there are some network issues going on?
This also sometimes happens when trying to edit a previous comment (takes a long time before the slanted gray bars disappear) and often when using the "View history" tab of Flow-enabled pages. I've not seen this happen on any non-Flow related pages on any Wikimedia sites.