ICMP messages are considered low priority within Junos OS, so the routing platform will respond to and process other higher priority messages, such as routing updates, before processing the ICMP messages. The microkernel may introduce tens of milliseconds of processing delay to ICMP message handling. The delay is not uniform, meaning that some ICMP messages might be delayed while others may not be delayed.
ICMP ping latency in traffic generated for the device itself is not a recommended way of testing the bandwidth or integrity of the link. Use other resources like transfering a file via FTP.