test2.sql

select r.DocEntry as rdrDocId,
        r1.LineNum as rdrLineId
into #orderLines
from rdr1 r1
join ordr r on r.DocEntry = r1.DocEntry
-- If I filter only by LineStatus or only by DocStatus here, query takes <1s
where r1.LineStatus = 'O' and r.DocStatus = 'O'

select U_KommNr as pklDocId,
       max(cast(U_Space as int)) as maxPlace
into #picklistDetails
from [@PICKING]
where U_DeletedF = 'N'
group by U_KommNr

select p.AbsEntry      as pklDocId,
	   -- If I remove the following reference to pd, query takes <1s
       case
       when pd.maxPlace is null then 0
	   else pd.maxPlace
       end             as pklDocMaxPlace
into #picklistDocs
from opkl p
left join #picklistDetails pd on pd.pklDocId = p.AbsEntry

select AbsEntry   as pklDocId,
       PickEntry  as pklLineId,
       OrderEntry as rdrDocId,
       OrderLine  as rdrLineId
into #picklistDocLines
from PKL1

select p.pklDocMaxPlace
from #picklistDocs p
join #picklistDocLines p1 on p.pklDocId = p1.pklDocId
join #orderLines r1 on r1.rdrDocId = p1.rdrDocId
                   and r1.rdrLineId = p1.rdrLineId

drop table #picklistDocs, #picklistDocLines, #picklistDetails, #orderLines