Is there anyway to get a speed slower than marathon?

In C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\Base\Assets\Gameplay\Data\GameSpeeds.XML, you can edit what each gamespeed's length is.

Making a game 3x the length of marathon:

Change this

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<Name>LOC_GAMESPEED_MARATHON_NAME</Name>

<Description>LOC_GAMESPEED_MARATHON_HELP</Description>

<CostMultiplier>300</CostMultiplier>

<CivicUnlockMaxCost>300</CivicUnlockMaxCost>

<CivicUnlockPerTurnDrop>30</CivicUnlockPerTurnDrop>

<CivicUnlockMinCost>60</CivicUnlockMinCost>

to

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<Name>LOC_GAMESPEED_MARATHON_NAME</Name>

<Description>LOC_GAMESPEED_MARATHON_HELP</Description>

<CostMultiplier>900</CostMultiplier>

<CivicUnlockMaxCost>900</CivicUnlockMaxCost>

<CivicUnlockPerTurnDrop>90</CivicUnlockPerTurnDrop>

<CivicUnlockMinCost>180</CivicUnlockMinCost>

 

and change this

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>180</MonthIncrement>

<TurnsPerIncrement>100</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>120</MonthIncrement>

<TurnsPerIncrement>300</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>60</MonthIncrement>

<TurnsPerIncrement>170</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>24</MonthIncrement>

<TurnsPerIncrement>201</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>12</MonthIncrement>

<TurnsPerIncrement>129</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>6</MonthIncrement>

<TurnsPerIncrement>180</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>3</MonthIncrement>

<TurnsPerIncrement>264</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>1</MonthIncrement>

<TurnsPerIncrement>156</TurnsPerIncrement>

</Row>

to this

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>60</MonthIncrement>

<TurnsPerIncrement>300</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>40</MonthIncrement>

<TurnsPerIncrement>900</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>20</MonthIncrement>

<TurnsPerIncrement>510</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>8</MonthIncrement>

<TurnsPerIncrement>603</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>4</MonthIncrement>

<TurnsPerIncrement>387</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>2</MonthIncrement>

<TurnsPerIncrement>520</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>1</MonthIncrement>

<TurnsPerIncrement>792</TurnsPerIncrement>

</Row>

 

-<Row>

<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>

<MonthIncrement>0.33333</MonthIncrement>

<TurnsPerIncrement>468</TurnsPerIncrement>

</Row>

Make sure you back up the config file before you do this, in case it doesn't work as you expect.

(If anyone wants to help with formatting to make this easier to read, feel free to comment)

/r/civ Thread