You are using recursion (using a AfterScan to call the Scanner continuously) .
Yes, that is to be expected. How often is this delay?
How can you speed this up? Perhaps you can’t. You might change your barcode scanner app on your device for another that might cycle faster; you could try a different device that has a faster cpu to process the data. If you are also printing results to a Label, stop that and only show the File results after your scanning session. One of the slowest processes in AI (even though writing to the screen is fast), it is not as fast as when using a java based compiler like Android Studio.
What is required of your device is to ‘take a picture’; convert that to numbers; post the results. A similar question might be is how fast can your device’s camera take a picture with successive camera presses. Notice, there is a delay. Your delay is probably related to the ability of the camera to provide a new image as a guess.
Functionality to be able to scan multiple qr codes at once? Not possible probably.
Consider, App Inventor 2 projects execute code asynchronously (one instruction after another); each time you issue a command, it takes a few ms to execute. These delays are additive.
You may not be able to use the scanner in your Android to be a High Speed Scanner. The link points to possible alternatives for you bicycle racing scanning activity.