difference in meaning

I gather you understand the meaning of prime as at its best.

1: Yes, he is a good actor. But now he is past his prime.

Past his best. It may be only a short time since he has acted at his best It is left to the listener to subjectivly decide just how many days/years it is since the actor was in his prime.

2: Yes, he is a good actor. But now he is way past his prime.

A long way (in time) past his best. A long time since he has acted really well.

By the way, I would say don't use full stop after actor because "but" is a joining word (conjunctive).
Yes, he is a good actor but now he is way past his prime.

If you wish you can use a full stop after "actor" just leave out the word "but".

Yes, he is a good actor. Now he is way past his prime.

Many thanks, Dadpad.

Just to confirm that I've got exactly what you said.

'past his prime' means 'no longer at his best'. We do not know how long.

'way past his prime' implies that he is no longer at his best since a long time ago'.

Thanks.

Past his prime' means 'no longer at his best'. We do not know how long.

Yes we do not know how long but a subjectively shorter time frame is implied.

way past his prime' implies that he is no longer at his best since a long time ago'

Yes, but subjectivly a "long time ago" or "way past his prime", to one listener may mean the actor has not done his best work for a year or two.
To another listener that may mean 5 or 10 or more years.

Thanks, Dadpad.

Using the word "way" as an emphasiser is very American. BrE would use a word like "decidedly" or "definitely" or a phrase such as "very much".