Noticed when upgrading from 4.0.0 to 4.0.3, 4.0.4 and 5.0.0 are affected too. Test case is a database with one big table. The size of the database is 3.3 GB:

Preparation, please adjust path and port as needed:

create database 'D:\temp\res.fdb' page_size 16384
user 'sysdba' password 'masterke'
default character set ISO8859_1;

create table bigone (
  F1 VARCHAR (1000),
  F2 VARCHAR (1000),
  F3 VARCHAR (1000),
  F4 VARCHAR (1000),
  F5 VARCHAR (1000),
  F6 VARCHAR (1000),
  F7 VARCHAR (1000),
  F8 VARCHAR (1000)
);

execute block
as
declare i int = 0;
declare n int = 15000000;
begin
  while (i < n) do
  begin
    insert into bigone values ('XXXX', 'XXXX', 'XXXX', 'XXXX', 'XXXX', 'XXXX', 'XXXX', 'XXXX');
    i = i + 1;
  end
end;

2. backup res.fdb

3. gbak -rep -page_size 16384 -se localhost/3054:service_mgr d:\temp\res.fbk d:\temp\res.fdb

Results:

FB  duration disk write  diff   comment

400 3:00     25           0     reference
403 3:49     20           +27%
404 3:44     20           +24%
500 3:34     13           +19%  ParallelWorkers = 1
500 0:55     52           -70%  ParallelWorkers = 6

duration is [min:sec],
disk write speed as observed in task manager is [MB/s], typical value, stabilizes after about one minute
diff is difference in duration vs 4.0.0

4.0.4.2993, 5.0.0.1215, the other two are official releases.