Se the above mentioned script - created by @cscs and @Kresimir - the script is well documented.
No matter how to approach this - the time taken is the time it takes whether it is during the operation or after the operation. The operation will not be faster if the sync is ongoing - your real issue is impatience - the system works as fast as the system can.
As already noted by @Aragorn you can use the sync option
sync All I/O to the filesystem should be done synchronously. In
the case of media with a limited number of write cycles (e.g.
some flash drives), sync may cause life-cycle shortening.
You can use systemd to mount an USB device to a specific mount point and use the sync option. As pointed out the device may have a shorter life-cycle if the option is used.