Carea more about big prey than you, so try to run past anything bigger and meatier than you, then keep running. It won't take long to kill whatever it finds. That being said, I did see a Rex take a long while to kill 2 carbonenemy turtles
The bread and butter of the Rex is their high Health. A good way to find a high Hp Rex and know it's Hp stat before KO is to ride a mount and use the magnifying glass to see it's health. Griffins, Sabers, and direwolfves are all good mounts to ride while trying to use the magnifying glass on the Rex. Between 7000 and 10000 is what you want to aim for to make it worth taming.
Being a raft pirate on me trusty raft with old Polly IV the Ichthyornis upon me shoulder, I a travel by the peak of far's ev'r now n' then, and aye see these big mates on the beach, a mosysaur of teh land. Grap'l onto yer flying beast n' fire at em er stay on ye bloody boat.
My friend and I tried to tame a 174 trek Rex with awesome colors, we tried landing on a medium sized hill but we didn’t know that Rex’s were mountain climbers back in the days so watch out, they will climb anything that doesn’t go exactly vertical