Hi Phil,
What you are asking is more improv related than speed related I believe. Speed will come with time when you practice enough, and when you really want to shred, working with a metronome to develop speed will work perfectly fine.
But to come up with improvisations on the fly you must develop a library of licks and riffs to be able to comfortably improvise. When you have those licks and riffs at your disposal it will be easy to use the scale and make it your own. It becomes also much easier to come up with variations of those licks and riffs.
Here at GuitarTricks you have the Blues courses and Rock courses in the Core Learning Systems.
Rock level 1:
https://www.guitartricks.com/course.php?input=rock1Blues level 1:
https://www.guitartricks.com/course.php?input=blues1Both these courses teach you the basics, and even include improvising. Both are EXCELLENT courses! Anders does a great job of teaching you some licks and how you can use these licks to improvise and above all make them your own (so you can make up your own variations). After that the level 2 courses go more in depth and are more advanced.
I really recommend these courses. You can start with whichever genre you prefer. These courses are the best I have come across on the web.
Hope his helps,
JJ