Query hangs (and then timeout) after using COPY to import data

Hello,

I'm new to PostgreSQL. I'm trying to migrate an app from SqlServer to
Postgres (it's written in C# and uses Npgsql)

I've tried with Postgres 16.1 and 16.2 on Windows Server 2019.

This app used SqlServer's Bulk Insert to import some tables (about 50
tables) from another database, I replaced it with Postgres' COPY function:
this part works correctly.

After the import, I execute sequentially (not in parallel) some queries in
these tables, to update some data and to make some validations.

At some point, systematically, one of these queries hangs, and after 10
minutes (the CommandTimeout that I set) it throws this exception:

Exception while reading from stream ---> System.TimeoutException: Timeout
during reading attempt

at Npgsql.Internal.NpgsqlConnector

The query is this one:

SELECT Id FROM Item

WHERE Id NOT IN (

SELECT ItemId FROM ItemUom)

LIMIT 100

The same query, executed from pgAdmin, returns the result in less than a
second (even if it's executed while the query from my app is running).

(actually the result are 0 record, but it's correct: the query it's just a
validation that there are no records in that query)

While the query is running from my app, I noticed that the CPU goes beyond
95%, even up to 100%, due to 3 postgres.exe processes.

The RAM usage is less than 70%.

In pgAdmin I've executed a query to list the running queries, and I can see
that one.

My issue seems to be very similar to this one:
https://stackoverflow.com/questions/77195107/npgsql-timeout-during-reading-a
ttempt
but I didn't understand how that guy solved the problem.

If I import less tables from the external database, the query doesn't hang
and runs correctly, so this make me think about some resources that could
"finish", but I haven't understood which one (for example the connections
used to import the tables and all the commands and datareader used to
execute the queries seem disposed correctly).

I don't know if it could be due to some Postgres parameter.

Do you have any suggestions to solve this problem?