Almost always, heat in a connector is caused by poor connection resulting in heat, which then reduces the pressure on the connection resulting in more heat etc, etc.
Most of those 6mm quick-connect style connectors are rated at 20A max (that's the connector, not the cable) but it is very hard to find any definitive specs for even the good quality units - the cheapies would be anybody's guess.
Not really a good place for this type of connector. I would definitely get rid of it and either use something much higher rated, or just do a good soldered and well insulated joint.