Linux web server is very slow and using 100% CPU

Solution 1:

a month ago I had a miner on my server (kdevtmpfsi). However, it was professionally removed by an IT security specialist.

It seems possible they might not have done a very professional job. Or you and they did not fully address the vulnerability which was previously exploited.

./linuxsys looks suspicious.

See How can I remove malicious spyware, malware, adware, viruses, trojans or rootkits from my PC?

Once a webserver has been compromised you really need to wipe it clean, reinstall and configure the operating system, reinstall your applications and restore data from one of your daily/weekly/monthly backups after verifying they are completely clean.