Using the dconf editor (run dconf-editor once installed), you'll want to edit the property called mousekeys-max-speed located at org.gnome.desktop.a11y.keyboard. From there, disable (uncheck) the Use default value box, and edit the Custom value field. Finally choose the [Apply] button for the new values to take effect.
BTW, as a bonus, while in the dconf editor, search for the term mousekeys and you'll see several other properties that you can edit to get the kind of functionality you're looking for.
Alternatively, if you don't want to install dconf-editor and stick to the CLI, you can use the gettings command to do your bidding. For example, to query the current mousekeys-max-speed key value, you'd run gsettings get org.gnome.desktop.a11y.keyboard mousekeys-max-speed, which should return 10 (the default value). Similarly, you'd use gsettings set to set an appropriate value. Additional examples and a more in-depth explanation of the gsettings command is available at the Ubuntu Manpage for gsettings.