Mavic crosstrails, bought secondhand. Installed, but when I freewheel the back wheel, he pedals turn slowly.

Assume this is friction in the freehub – is this easily servicable or a replace job?

they all do it. it’s friction on the freehub seal.

2 things to watch out for…
the freehub and chain wanting to continue going round when freewheeling, and the mavic deathscream.

mine was fatal in the end, and a very slight bend in the axle making it not economical to replace the seal bushing.

was a thread on here just a few days ago about the scream / wail.

it’s very very easy to take apart (older models at least), and clean, lube etc.

Is there a permanent fix?

I assume therefore when I am freewheeling the hub is introducing friction into the system and so I am losing energy.

Grr

Yeah I cleaned the rubber seal and light lube stops it

The permanent fix is to keep to mavics servicing schedule, freehub should be stripped and oiled at least twice a year, would probably recommend 4 times on a mountain bike.