API call limit - introduce delay

There's no easy way of tracing this in Power BI; you could use a tool like Fiddler though. 

It is possible that multiple calls are being made. It might be worth experimenting with Table.Buffer() here (see https://blog.crossjoin.co.uk/2015/05/05/improving-power-query-calculation-performance-with-list-buffer/ for some background). Maybe something like this would work:

let
   Source = ()=> Json.Document(Web.Contents("https://api.skulabs.com/purchase_order/get?limit=100&skip=&sort=")),
   #"Converted to Table" = Table.FromList(Function.InvokeAfter(Source, #duration(0,0,0,2)), Splitter.SplitByNothing(), null, null, ExtraValues.Error),
   #"Expanded Column1" = Table.Buffer(Table.ExpandRecordColumn(#"Converted to Table", "Column1", {"number", "distributor_id"},          {"Column1.number", "Column1.distributor_id"})),
   #"Merged Queries" = Table.NestedJoin(#"Expanded Column1",{"Column1.distributor_id"},MapTable,    {"Column1._id"},"NewColumn",JoinKind.LeftOuter)
in
#"Merged Queries"

Chris