one thing about increasing speed is that u have to relax ur hand. if your hand is tense motion will be restricted by the resistance in your hand. you have to try to be really relaxed, obviosly this is easy at slow speeds so just do some easy licks slowly and speed them up but make sure your hand is relaxed. i had the same problem myself, cos ur hand ends up getting tierd quickly. otherwise its just practice. if its purly a speed thing i would sugest choosing three notes and playing them quickly with relaxed fingers and making different patterns. if it is the riff itself, eg it is tricky eg string skipping, just paractice it a lot.

anyway good luck and keep practicing. i found developing speed takes time so u mite want to develope some other skills at the same time.