The working theory is that "tcp_collapse" causes spikes of ~20 ms, which in turn make Linux not receive packets in time, which makes the network buffers to fill

The working theory is that "tcp_collapse" causes spikes of ~20 ms, which in turn make Linux not receive packets in time, which makes the network buffers to fill, which means new packets will be lost, which may be visible as 1s... latency spike end-to-end. This is for the TCP case. For the ICMP, indeed, the 1.8s times are not explained, but it's totally possible that 100 of 18ms events accumulated. Remember that the ping was running for 32 hours, while the system tap scripts for 30 seconds.