This is not an answer out of the box, with all client/db stuff you may need to do some work to determine exactly what is amiss
backup postgresql.conf changing
log_min_duration_statement to 0
log_destination = 'csvlog' # Valid values are combinations of
logging_collector = on # Enable capturing of stderr and csvlog
log_directory = 'pg_log' # directory where log files are written,
log_filename="postgresql-%Y-%m-%d_%H%M%S.log" # log file name pattern,
debug_print_parse = on
debug_print_rewritten = on
debug_print_plan output = on
log_min_messages = info (debug1 for all server versions prior to 8.4)
Stop and restart your database server ( reload may not pick up the changes )
Reproduce your tests ensuring that the server time and client times match and that you record the start times etc.
copy the log file off an import into editor of your choice (excel or another spreadsheet can be useful for getting advance manipulation for sql & plans etc)
now examine the timings from the server side and note:
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is the sql reported on the server the same in each case
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if the same you should have the same timings
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is the client generating a cursor rather than passing sql
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is one driver doing a lot of casting/converting between character sets or implicit converting of other types such as dates or timestamps.
and so on
The plan data will be included for completeness, this may inform if there are gross differences in the SQL submitted by the clients.