There are certain rudiment combinations that force you to shift lead hands- mid-pattern.
Adding your own accents to these exercises will build up your weak hand in no time.
GREAT EXERCISE FOR BUILDING LEFT HAND:
Paradiddle/6Stroke Roll: Start paradiddle at 1 count. Start 6 stroke roll at 4 count. Back to paradiddle at 1 count.
With this pattern, your lead hand changes on every 1 count. Build this up to speed and add it to your exercise routine. You WILL get faster.
That's just one idea.
For more training on the topic of building insanely powerful HAND SPEED quickly, go here:
For a stockpile of drum fill ideas, check out this youtube channel:
To build insanely powerful FOOT SPEED quickly, try this training pack: