Difference between 'sync' and 'async' mount options

async is the opposite of sync, which is rarely used. async is the default, you don't need to specify that explicitly.

The option sync means that all changes to the according filesystem are immediately flushed to disk; the respective write operations are being waited for. For mechanical drives that means a huge slow down since the system has to move the disk heads to the right position; with sync the userland process has to wait for the operation to complete. In contrast, with async the system buffers the write operation and optimizes the actual writes; meanwhile, instead of being blocked the process in userland continues to run. (If something goes wrong, then close() returns -1 with errno = EIO.)

SSD: I don't know how fast the SSD memory is compared to RAM memory, but certainly it is not faster, so sync is likely to give a performance penalty, although not as bad as with mechanical disk drives. As of the lifetime, the wisdom is still valid, since writing to a SSD a lot "wears" it off. The worst scenario would be a process that makes a lot of changes to the same place; with sync each of them hits the SSD, while with async (the default) the SSD won't see most of them due to the kernel buffering.

In the end of the day, don't bother with sync, it's most likely that you're fine with async.

Words of caution: using the 'async' mount option might not be the best idea if you have a mount that is constantly being written to (ex. valuable logs, security camera recordings, etc.) and you are not protected from sudden power outages. It might result in missing records or incomplete (useless) data. Not-so-smart example: imagine a thief getting into a store and immediately cutting the camera power cable. The video recording of the break-in was recorded but might not have been flushed/synced to the disk since it (or parts of it) might have been buffered in memory instead, thus got lost when the camera lost power.