"A Perfect Spy" is clearly a high quality production by the era's standards, with good performances and an authentic seriousness.
But it is slow. Really slow. And more crucially, it doesn't have enough story in it to justify 6 hours of screen time.
The scope is epic. Three actors play Magnus Pym - as a boy, a young man, and an older man - over the course of 50 years of his life.
First we're introduced to his father, Rick Pym, a charismatic character who skirts around the edge of honesty. The younger Pym seems to have picked up his father's economical way with the truth. As a young man, he helps his father out, but ends up having to find his own way in cold-war Europe when things go wrong, making unusual friendships along the way. As an adult, he builds a respected career in the secret service. But all is not as it seems.
The story of a Pym's deception is interesting enough. But the big problem here is that you never get the sense of any actual spying being done. It's a sequence of conversations, and coded ones at that (British people of the era were bad at talking directly, so spies are almost incomprehensible).
As a drama it would have been so much more convincing if it had focused on the impact of the spying, the moments where there were breaches of trust, and Pym's motivation (which is never clear).
Instead it plays out more like a memoir of a man with a dodgy Dad who was a bit dodgy himself. This may be true to Le Carre's book (I haven't read it) - but it doesn't make for good TV drama.