The one I've got makes a really fast job of a light cut. But whenever it
meets something meaty it just stalls - which happens annoyingly enough,
especially during the winter cut back (I try not to trim much in summer
because of the birds nests)

I just borrowed another - and it's the same.

Both of them move their cutters really fast - then stall on anything big

Does anyone make one with a lower gear? I'd accept a couple of minutes
more on the hedge so I didn't have to keep stopping and getting the
secateurs out.

Must be battery.

Andy