Ok, many thanks to @helpdesk I think we at least see what’s going on here (I suspect this also covers many of the others in the thread)
The way our website is currently architected, each Pi/Arduino device creates a websocket connection. In this particular account, there were probably ~30 Pis, which equals 30 websocket connections, which equals 
(at least that’s what happened to mine – runaway CPU consumption 
)
We’ll be replacing this architecture soon with one that only uses 1 websocket per browsing session, (the same that we use for MQTT and LoRa devices). In the meantime, there are a few potential workarounds:
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As mentioned above, this issue does not occur with MQTT devices, and both Pis and Arduinos can be connected via MQTT. So one option would be to remove ‘traditional’ Raspberry Pi and Arduino entries and re-add them to the dashboard as MQTT devices.
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Until this is fixed, another option is to split your devices between multiple accounts. I generally run with 8-10 devices on my account without much slowdown – I suspect this will come down to system specs more than anything else.
If @helpdesk or anyone else in this thread needs help with removing devices from your account so you can re-add them via one of these methods, but can’t because of the slowdown, please let me know and I can assist, and thank you for helping us find this architectural issue.