In the Wound Healing report, under the 'Overview' section, each well is compared in Scratch Closure Speed. This speed is something of an average which is determined by the following steps:

find the point in time in which the scratch covered area reached its peak value, this will be the end time.

find the delta area between the end time to the first frame.

divide the delta area by the time difference to calculate the average speed.
In the 'Well Details' section of the report, the average speed over each time interval is given in terms of percent closure over time. The calculation is done as follows:
delta[t=1] = OpenArea[t=0]/GapArea[t=0]  OpenArea[t=1]/GapArea[t=1];
speed = delta[t=1] / intervaltime
So that speed is positive as open area decreases.