Congrats on having a disco! I have a '97 and use it every day for a 40 mile commute. The main reason I got it was for that occasional trip in sand I like to make, but it's a great commute vehicle too.
In answer to your question I wonder if you are using premium gas. Not much about land Rovers is inexpensive, and gas is pretty pricey-even for regular. I never tried premium, but it's recommended. If you're not using it try it and see if that helps. Maybe you mean some other kind of performance. When you get ready for new tires maybe some designed for what kind of performance you want might help. The difference in design, or tread may make a difference at no different cost.
My next tire will be for better performance in sand, although my disco gets me there and back with the tires I got with it.
Good luck,
Larry