Turns out I had missed this page from the docs:
https://docs.apify.com/platform/actors/development/programming-interface/system-events.
The websocket sends 2 events (the second one is not documented though):
{
"name":"cpuInfo",
"data":{
"isCpuOverloaded":false,
"createdAt":"2024-06-05T09:16:37.916Z"
}
}
{
"name": "systemInfo",
"data": {
"memAvgBytes": 127389235.71674345,
"memCurrentBytes": 168525824,
"memMaxBytes": 257417216,
"cpuAvgUsage": 1.655466795311761,
"cpuMaxUsage": 115.72125786163522,
"cpuCurrentUsage": 7.616518557239937,
"isCpuOverloaded": false,
"createdAt": "2024-06-05T09:16:39.092Z"
}
}