That's totally normal behaviour. Actually the wing is so thin that the aicraft is hard to slow. Also the B777 has a lot of inertia, hence it is more difficult to slow down. That's why it's very important to well prepare your descent schedule and to make sure your on profile with this type of heavy airplanes.
For deceleration it's a bit like the NGX, which also has a thin wing section.
Good luck ! Use sound judgement and airmanship. To make sure you're on profile you can always x-check the VNAV with an easy formula with use on the big airliners : ALT to loose X 3 = distance required.
Or you will loose 1000 Ft in 3 NM.