Hi! To improvise effectively, learning your fingerboard is essential. Learn the intervals of each and every scale you`re going to use. An excellent starting point is to begin and your own improvised line on a chord tone and end it on a chord tone of the subsequent chord. By that I mean, start for example on the root of an underlying chord, play a melody that you like and end on an chord tone of the next chord. For instance, when you improvise over G major and D major chords, start on G (a root) of a G major chord and end your phrase on say F# (3rd) of the D chord. It takes time to get used to it and eventually the speed will come too. :)