Increasing parallelism of queries while using file fdw and partitions

Wondering if anyone had any thoughts on how to tweak my setup to get it 
to read many files at once instead of one at a time when using file fdw 
and partitions. We have a bunch of data tied up in files (each file > 4M 
rows, 5,000+ files per year) that I would like to be able to query 
directly using FDW. The files are genomic VCF format and I find that 
vcf_fdw ( https://github.com/ergo70/vcf_fdw ) works really well to read 
the data. We only want to be able to search the data as quickly as 
possible, no updates / deletes / ...

I gave an example below of the basic setup and the output of explain 
analyze. I get the same performance if I setup the table such that the 
thousands of files end up in one non-partitioned table or setup each 
file as it's own partition of the table.

I have tried increasing ( / decreasing ) the worker threads and workers, 
but don't see any change in the number of files open at any given time. 
I tried reducing the cost of parallel queries to force them to run, but 
can't get them to kick in.

Any ideas or anything I can try?

Thanks!

Pat

PostgreSQL:  PostgreSQL 10.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28), 64-bit
Multicorn: 1.3.5
VCF_FDW ( https://github.com/ergo70/vcf_fdw ) : 1.0.0


CREATE DATABASE variants;

CREATE EXTENSION multicorn;

CREATE SERVER multicorn_vcf FOREIGN DATA WRAPPER multicorn OPTIONS (wrapper 'vcf_fdw.VCFForeignDataWrapper');

CREATE SCHEMA vcf;

CREATE TABLE vcf.variants ( ..., species text, ... ) PARTITION BY LIST ( species );

CREATE FOREIGN TABLE vcf.human ( ... ) SERVER multicorn_vcf OPTIONS (basedir '/path', species 'human', suffix
'.vcf.gz');
ALTER TABLE vcf.variants ATTACH PARTITION vcf.human FOR VALUES IN ( 'human' );

CREATE FOREIGN TABLE vcf.dog ( ... ) SERVER multicorn_vcf OPTIONS (basedir '/path', species 'dog', suffix '.vcf.gz');
ALTER TABLE vcf.variants ATTACH PARTITION vcf.dog FOR VALUES IN ( 'dog' );

CREATE FOREIGN TABLE vcf.cat ( ... ) SERVER multicorn_vcf OPTIONS (basedir '/path', species 'cat', suffix '.vcf.gz');
ALTER TABLE vcf.variants ATTACH PARTITION vcf.cat FOR VALUES IN ( 'cat' );

* My real data repeats this 1000+ more times

EXPLAIN ( ANALYZE, BUFFERS ) SELECT * FROM vcf.variants WHERE chrom = '1' AND pos = 10120 LIMIT 1000;

On my real data I get the following results:
--------------------------
                                                                                  QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Limit  (cost=20.00..352020.00 rows=1000 width=347) (actual time=445.548..101709.307 rows=20 loops=1)
    ->  Append  (cost=20.00..3555200000.00 rows=10100000 width=347) (actual time=445.547..101709.285 rows=20 loops=1)
          ->  Foreign Scan on dog  (cost=20.00..3520000.00 rows=10000 width=352) (actual time=198.653..198.654 rows=0
loops=1)
                Filter: ((chrom = '1'::text) AND (pos = 10120))
          ->  Foreign Scan on cat  (cost=20.00..3520000.00 rows=10000 width=352) (actual time=111.840..111.840 rows=0
loops=1)
                Filter: ((chrom = '1'::text) AND (pos = 10120))
          ->  Foreign Scan on human  (cost=20.00..3520000.00 rows=10000 width=352) (actual time=135.050..138.534 rows=1
loops=1)
                Filter: ((chrom = '1'::text) AND (pos = 10120))
          ... repeats many more times for each partition
  Planning time: 613.815 ms
  Execution time: 101873.880 ms
(2024 rows)