It also depends on your data plan. Pretty much any data plan that bills itself as unlimited will throttle at some data cap or other criteria. If you have a straight up data limit, then you usually get full available speed up till you get nothing.
Keep in mind that full speed depnds on your connectivaity, which can vary a lot depending on the area, signal strength, and how congested the local cell towers are at that time.
That said, it’s good to have a fast wi-fi network wherever you spend the most time with your iPad. Especially if you stream video. Automatic backups and much of the background syncing also require a network connection (with the iPad inactive and plugged into power).