I can’t run your code, but you can take advantage of the max and min functions also returning the index of the vector at those values:
[min_velocity,minidx] = min(velocity);
[max_velocity,maxidx] = max(velocity);
avg_velocity = mean(velocity);
avgidx = find(velocity <= avg_velocity,1,'last');
Since the mean may not equal any element in the vector, getting the last element less than or equal to the mean is likely as good as it gets.
To get the times at those values, reference them by index:
t_minv = time(minidx);
t_maxv = time(maxidx);
t_avgv = time(avgidx);
