Good encryption performance
It's probably fastest to use COPY (or \copy) to copy the data into unencrypted tables. Disable triggers, foreign keys, indexes. 

Then encrypt, and add your indexes and keys again. 

Use one SQL operation to encrypt, like

update my_table set 
    bytea1 = pgp_sym_encrypt_bytea(bytea1,var_pwd),
    bytea2 : = pgp_sym_encrypt_bytea(bytea2,var_pwd)
    ....;


Or do a insert into...select into a new table, encrypting in the select. 

See also https://wiki.postgresql.org/wiki/Performance_Optimization