I have been creating A records for subdomains lately, and found that the TTL on the A record matters, but even more important for fast response time is the minimum time to live (reverse cache) on the SOA record. Setting the A record to a TTL of 1 hour and the minimum time to live on the SOA of 1 minute results in almost instant record propagation. Not exactly sure what the SOA minimum time to live even does though.