If you are flashing a file to the micro:bit via USB using the drag & drop process, it may take a little longer than usual. 

To enable us to flash a board to any micro:bit, the micro:bit editors now support a Universal Hex format, which contains code for both previous and latest micro:bit revisions and so the compiled .hex file size will be ~1.8Mb as opposed to ~700Kb in size.

This results in a slight increase in flashing time when using drag and drop.

If your micro:bit is using firmware 0234 please try upgrading it to the latest firmware version for your board.

However, if you have a Chrome-based browser, like Google Chrome or Microsoft Edge, you can use direct flashing from the browser to update your programs. This process only updates the portion of the program that has changed, so once you have flashed your program from the browser, subsequent updates will be much quicker.