This stop is done gracefully in the background so it may take a couple of seconds between clicking on a button and actual task abort.
This is because SyncSpider stops task in a proper way.
SyncSpider performs operations in data manipulation internally and with external systems. Forced abort could leave some systems in the unpredicted state, and some data could create junk if not properly handled. Hense the delay.
If the task is stopped (aborted) by a user, this will be displayed in execution log.