Bulk Import Taking too long

Hey there guys. I'm trying to import data from a csv file with about 300K rows. The query I am using is as follows:

CALL apoc.periodic.iterate(

'CALL apoc.load.csv("file:///newfile.csv") Yield map as row return row'
'MERGE (s:Sender{from_send:row.From}) 
MERGE (r:Receiver{to_send:row.To}) 
MERGE (s)-[e:EMAILED{date: row.Date, time:row.Time, subject:row.Subject, message_id : row.MessageID}]->(r)'
{batchSize:20000, iterateList:True, parallel:true}

But this query seems to be taking forever.(looks like its running in an infinite loop- kept running for 15 min)

Note that there are blank values in the "Subject" column in the csv as well. Can anyone please tell me what's the problem. Thanks

Try with parallel:false and ensure you have an index or unique constraint on Sender and Receiver.

should there be unique constraints on sender/receiver since a single sender can send emails to multiple recipients? Please correct me if I'm wrong or couldn't understand your point.
Thank you

Any node being used in a MERGE should be backed by an index or unique constraint. Otherwise the MERGE will get massivly slower with data growth.