Hi,
if you read the release notes for 15.1X49-D70 (http://www.juniper.net/techpubs/en_US/junos15.1x49-d70/information-products/topic-collections/release-notes/15.1x49-d70/junos-release-notes-15.1X49-D70.pdf) under resolved issues, the following PR is seen:
• On SRX300 Series devices, high RE CPU is noticed when ethernet-switching is configured. This leads to a higher than expected latency for traffic originating from the device itself. This issue has no impact on transit traffic through the device. PR1206823
It could sound like the issue people are facing.
15.1X49-D70 release notes are online but the actual software image is not. I would expect to show up shortly as D70 is expected to be release this month.