Hi
From my experience of the mac has gotten really slow all of a sudden that tends to mean that ether the SSD or RAM is on its way out
Just to be in the safe side I would back up your data and try the following
Create a partition on your Mac to install macOS onto to see if that resolves the issue
I would also try running the hardware diagnostic
[link|https://support.apple.com/en-gb/HT202731|https://support.apple.com/en-gb/HT202731|new_window=true]
If you don’t get any issues I would consider replacing the SSD first then the RAM if the issue persists
SSD’s degrade over use just like HDD’s do,they will all need replaced eventually
Hopefully this helps
Any questions please ask
Thanks:-)