[GianFabien]

My MBP is over 10 years old and runs just fine.

Have you looked at the processes, there might be some service that has become a hog.

Another possibility is that you have written a lot files to your SSD and/or your filesystem is near full. Another issue might be excessive swapping. Once the wear levels or saturation on the SSD hit a certain threshold, they can become very slow.

[npollock]

thanks - is there a way to determine/monitor if excessive swapping is taking place?