Was the Mac running acceptfully fast previously? Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?
Before sending money on hardware you should confirm it's not software that's causing the slowness. Therefore, download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.
Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.
