Malware

If it was just getting hot, I'd say maybe a battery issue - or even a fan issue (you'd be amazed what a bit of compressed air into the fan exhaust can do to blow out dust causing heat problems).

With heat, spontaneous shutdowns and screen issues...I'd suggest a trip to the Genius Bar if your model is still supported - might be a logic board about to fail or some other issue better suited to diagnostics and/or repair.

if you did not allow them access, and did not provide your admin password, your slowness and overheating are not due to spontaneous attack.

Consider downloading and running this little "discovery" utility, Etrecheck. It changes NOTHING. Etrecheck was developed by senior contributor here, and uses system calls to collect often-needed information.

it contains little tests for speeds of devices, CPU utilization, memory usage, energy usage and a digest of recent problems, in one easy to use package. it does not even need to be Installed. Because less can be learned when your Mac is running great, best time to run is when your problems are actually occurring, if possible.

if you follow the directions faithfully, its report (pre-laundered of all personally-identifiable information) can be "Shared" to the System ClipBoard, then Pasted into an ‘Additional Text’ window in a reply on the forums.

Use Etrecheck pro for free:

http://Etrecheck.com

...

Macs are not subject to spontaneous infection my malware and viruses. Nothing can be come executable unless/until you supply your Admin password to "Make it So".

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself. They are relentless in scanning your files, non-stop, looking for virus-like patterns in Everything, or looking for files that have changed. When completed, they do it all again.

do you have anything like that running?

Apple has always had antivirus software built into all their devices and has been at the forefront and industry leading in Safety and Security.

Mac starts with state-of-the-art antivirus software built in to block and remove malware. The chips in MacBooks have built-in Secure Enclave which protects your login password, automatically encrypting your data, and more from your files to data. Apple even helps you keep your Mac secure with software updates and security updates frequently.

The odds of your device being infected with malware are slim to none, unless you somehow received the malware not under MacOS and Apple system, for example third-party downloads and files. The odds are still very rare.

For more MacOS security information: macOS - Security - Apple