What version are you running? If you refer to this post there was an issue with some versions of 7.6.2 and 7.7.1
The tempfs is just a ram disk, not a hard disk partition. There's also a good chance that a process restart will temporarily clear the issue (or system reboot). I'm not 100% sure but I believe it's used as temporary storage/processing of update files and or AV related data.
You can run an htop from the command line as well and see if any of the processes are using an unusually high amount of Memory or CPU utilization, may identify the problem process.
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