If an indicator is set to have synchronous display the block diagram will always wait until the indicator value has been updated on the screen, otherwise everything will just continue even though the user interface has not "discovered" the new value yet....
here is an extract of NIs text on performance;
"In multithreaded systems, controls and indicators have a synchronous display pop-up item that controls whether or not updates can be deferred. In single-threaded execution, this item has no effect. However, if you turn this item on or off within VIs in the single-threaded version, those changes affect the way updates behave if you load those VIs into a multithreaded system.
By default, this item is off, which means that after the execution system passes data
to front panel controls and indicators, it can immediately continue execution. At some point thereafter, the user interface system notices that the control or indicator needs to be updated, and it redraws to show the new data. If the execution system attempts to update the control multiple times in rapid succession, you might not see some of the intervening updates.
In most applications, this significantly speeds up execution without affecting what the user sees. For example, you can update a Boolean value hundreds of times in a second, which is more updates than the human eye can discern. Asynchronous displays permit the execution system to spend more time executing VIs, with updates automatically reduced to a slower rate by the user interface thread.
If you want synchronous displays, you can turn the synchronous display on.
Note: Turn on synchronous display only when it is necessary to display every data value. Using synchronous display results in a large performance penalt
y on multithreaded systems."
