Not really, but they are grafted on to Hawthorn rootstocks, so maybe that is where the thorns are coming from. And that is a very slow rate of growth for a Medlar, even a grafted one should grow at about a foot a year minimum.

Mine is on a dwarfing quince (C, I think) rootstock and it is growing quickly. The common medlar varieties do not have thorns.
A sucker from a hawthorn rootstock would have thorns.

I agree with Owdbuggy, the tree should be much taller than this by now. If you have been pruning it then I would stop doing so for a bit.