Slow vacuum of GIST indexes, because of random reads on PostgreSQL 9.6

I am using a GIST index on timestamp range, because it supports 'contains'
operator ('@>'). Unfortunately, in large scale (billions of rows, index
size: almost 800 GB) vacuuming the index takes an order of magnitude longer
than btrees (days/weeks instead of hours).
According to the code, during vacuum gist index is traversed in a logical
order which translates into random disk acceses (function gistbulkdelete in
gistvacuum.c). Btree indexes are vacuummed in physical order (function
btvacuumscan in nbtree.c).

As a workaround, I'm planning to replace all uses of 'contains' with the
following function:

CREATE OR REPLACE FUNCTION tstzrange_contains(
range tstzrange,
ts timestamptz)
RETURNS bool AS
$$
SELECT (ts >= lower(range) AND (lower_inc(range) OR ts > lower(range)))
AND (ts <= upper(range) AND (upper_inc(range) OR ts < upper(range)))
$$ LANGUAGE SQL IMMUTABLE;

and create btree indexes on lower and upper bound:

CREATE INDEX my_table_time_range_lower_idx ON my_table
(lower(time_range));
CREATE INDEX my_table_time_range_upper_idx ON my_table
(upper(time_range));

Is it the best approach?

--
Best regards,
Marcin Barczynski