Choose a passage to practise. Alternate hands playing it. For every repeat of the RH do 3 repeats of the LH.
Keep in mind that most of the time, the left hand is less able then the right not because of “fingers” but because the whole left side of your body is less used. So your left hand may be slower because you are using only your fingers, while the right hand you are using your whole arm.
Think about bringing a fork to your mouth. It is not done with the hand – the hand just holds the fork. It is the arm that brings the hand (and consequently the fork) towards your mouth. But because the hand holds the fork and our attention is on the fork (the food is there!), we concentrate on the hand and the arm movement happens unconsciously. This is all right with the right hand – you are used to it. But if you try to eat with the left hand, you become very clumsy first because you are concentrating on the hand, and secondly because your left arm is not used to the movement necessary to bring the hand over to your mouth.
If you observe carefully, you will notice that your right hand moves in a dramatically different way from the left when playing a similar passage (you must make sure it is a mirror image, since our hands are mirror-images; if you are playing an ascending scale with the RH you must play a descending scale with the LH otherwise your movement comparison will not be valid).
So here are two approaches:
1. Choose a passage and its mirror image. Observe carefully how you do it with the RH, and how you do it with the LH. Work on making the LH movements as similar as possible to the RH movements (assuming your RH is your better hand and that you are satisfied with its performance). This way, the RH “teaches” the LH.
2. Spend a month consciously doing all sorts of things with your LH (eating, opening doors, combing hair, using the mouse, you get the idea). This will get the left side of your body as adept at general movement co-ordination as the right.
Have a look here for discussions on left hand issues:
https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=repo;action=display;num=1084390696https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=stud;action=display;num=1085018502https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=teac;action=display;num=1049478047https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=stud;action=display;num=1036101959;start=0Also have a look here, where speed has been discussed at length:
https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=stud;action=display;num=1087278993Just the tip of the iceberg.

Best wishes,
Bernhard.