View crossing multiple databases, but a query only using one is slow
Never mind,

I just went with sp_executesql

Performance issue are gone, readability gone

SET @myInst = '[db2Name]'
SET @sqlReplaced = REPLACE('myQry','{@myInst}',@myInst)

EXECUTE sp_executesql
@statement                  = @sqlReplaced,
@ParmDefinition             = @ParmDefinition,

@clientNo                   = @clientNo,
@lastTransactionDate                = @lastTransactionDate,
@msInstanceNo               = @msInstanceNo, 
@dateFrom                   = @dateFrom , 
@dateUntil                  = @dateUntil