Updating database from a really large CSV

I have a database with about 30K nodes. For simplicity, let's suppose it looks like

appID
1
2
3

I want to update my database with information from a much larger csv file (19M rows) that looks like

appId userID
1 400
2 450
5 300

So, the end result will be

appId userID
1 400
2 450
3

May I get help in how to do this efficiently? I've tried the below, but it is taking a very, very, very long time (like, nearly infinite).

Many thanks.

LOAD CSV WITH HEADERS FROM 'file:///otherFile.csv' as row
CALL {
with row
match (a:application {appID: row.appID})
merge (u:UserId {userId:row.userIs})
merge (a)-[:connectsTo]->(u)
};