Slow system due to ReorderBufferGetTupleBuf?

Postgres v10 on Debian stretch

I’m suffering from an occasionally very slow system. A few weeks ago someone mentioned using perf. I’ve installed this
andcaught the system during a slow period. It shows the following as the top cpu users: 

9.09%  postgres                      [.] ReorderBufferGetTupleBuf
6.14%  postgres                      [.] ReorderBufferReturnChange



When ReorderBufferReturnChange is no longer running:

14.35%  postgres                      [.] ReorderBufferGetTupleBuf


Can someone shed some light on this and advise how to prevent it reoccurring?


Cheers,

Martin.