I set up a test channel on Slack and hooked it up to the Hacker News "firehose" feed, which is fairly high volume. About 20 or 30 posts per hour. I wanted to see how fast it would update.
I also wrote a small Node.js app that reads the same feed and posts to another Slack channel, as a control. It will post at most one item per minute. So they come out fairly steadily as long as more items are being posted to Hacker News.
I have also set up an IFTTT recipe to read the feed and see how it compares.
It took about 1/2 hour for IFTTT to push a dozen items to our group at once.
Another 1/4 hour another storm of updates from IFTTT.
Still nothing from Slack's RSS bot.
My Node app is chugging along, posting one item a minute.
It's been over an hour and still nothing from Slack.
We are now getting updates from Slack's native RSS support.
They do more than report headlines, they're scraping the articles that are being linked to from the HN firehose feed.
But they're doing that with some of the links we push from the Node.js app.
And now it's been a couple of hours since the IFTTT bot has posted any new stuff, yet the feed is updating continually.
Then as if by magic a small flood of IFTTT posts from the feed.
Early conclusions: Slack's native integration is slow and bursty. IFTTT is more regular but bursty. Lots of messages all at once. Neither is ideal for a frequently updating feed.