I wrote this a while back, it’s my personal checklist for looking into performance issues and specifically slower-than-desired KV ops: Couchbase performance issue - slowness - #4 by graham.pople. Maybe that’ll provide some pointers to help you diagnose this, particularly the second half of the checklist which is more centred on diagnostics.
(I know you’re reporting timeouts rather than slow op issues, but there’s a lot of overlap of course between the two - increase your KV timeouts to 10 minutes and now you have the latter rather than the former 
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