MacBook Pro (Retina, 15-inch, Mid 2015) slow after recent security update

A few days ago I updated my MBP with the security update for Safari and the system.

Since then, it has slowed down. At the start up it does not read an external disk which I use daily (I have to unplug it and plug it in again).

The only thing I have come up with is just a restart and a PRAM reset.

No improvement.

I would be grateful if anyone could help me with that.

Thank you.

Paolo