Interesting question. I would say it depends how much room you can give them, and also how spooky they already are. I would say generally 10-30mph, a higher speed when you can give the most room.
Cars do pass me much faster than that, but both mine are very good in traffic, so I don't mind too much provided they aren't too close.
Even if there was an acceptable speed by law, not many would observe it. How many do I see observe our road's 30mph?? Hardly anyone!!! There are two primary schools along it, lots of yards and horse riders up and down especially weekends...
MOST people are very considerate and slow right down. Other pass as 30mph and give me a wide berth. Some just don't care.
You can't control other people. You can only control how you/your horse reacts to traffic. This means more exposure over time. Accidents DO happen and it is a horrible truth no matter how perfect you and your horse are in traffic.
I can stand and chat to farmers in noisy tractors with my horse more interested in the hedgerow picnic but who's to say a boy-racer/BMW driver isn't going to come speeding around a corner and kill us both in a split second?
I tend to drive about 10 past horses but I did have a mare that drivers could go past at normal or fast speed and she didn't bat an eyelid but if drivers slowed down she would start pratting about.