As others have said the answer is situational. There are roads here where 10mph is too fast.
The fastest I ever went was two summers ago on a steep descent where the road was freshly paved - it was a thrill, can't remember the exact speed but it was about 45.
I did that same road last summer but after I had thought back to the speed I hit the previous year and the consequences that might have happened if I pulled a flat or a small mammal jump out in front of me - I slowed down. After nursing a broken collar bone for 8 weeks I think I'll slow even more when I do that descent this summer. I guess I am just getting older.

For me the goal is to improve the speed going up, not the speed going down.