Issue
I have some mappings that have the Accelerated Polling option enabled, and they no longer seem to be working. The "Streaming poll found n changes" messages that I used to see in my logs aren't present anymore. Data sync from periodic polling at my mapping's polling frequency is still working.
Resolution
Heroku Connect uses Salesforce's Streaming API to listen for changes to an object in Salesforce, rather than having to wait for those changes to be seen in the next polling period.
However, Accelerated Polling operates on a best effort basis and data synchronization might fall back to the polling period (interval polling). This can happen if Salesforce is not emitting PushTopic events for the object or is emitting these with a delay, or due to temporary connectivity issues from Heroku Connect to Salesforce.
A workaround to get the accelerated polls and streaming updates working again is to re-authorize your Salesforce connection with Heroku Connect.