What will be best (for performance) transaction isolation level?
READ COMMITTED imposes fewer overheads as PostgreSQL does not have to do dependency tracking.

In practice the difference is likely to be minimal. 

Isolation level should be selected based on correctness, not performance. It's not possible to determine whether READ COMMITTED is sufficient from the queries provided, but I don't see a strong reason to suspect it not to be.

There are almost certainly many other ways to improve performance before playing with things like isolation levels anyway.