What you said
In our school teachers use stopwatches all the time - there is an online one. Where appropriate, you could introduce an element of competition to your tasks.
The expert view
From the tone of your message, I infer that there is a vital ingredient missing from the lesson: urgency, as defined by the desire not to fall behind due to penalties being incurred by the pupils.
There are two blunt ways to propel and modify a pupil's behaviour to meet what you desire: rewards and sanctions. Both motivate in their own ways, and both have their drawbacks, because they are tools that serve different jobs.
But telling them how disappointed you are simply is not enough. If that is the worst they incur for living in the land of do-as-you-please, then for many of them the mathematics is simple: do as they please because the penalties are so slight as to be ignorable.
Attach some serious consequences to your boundaries. Electrify the fence that surrounds your class (metaphorically), the one defined and designed by you, sure in the knowledge that you know what is best for them. In other words, if they fail to complete tasks as set by you within the time limits you have imposed and, crucially, you believe that they could have done much more, then the mantrap of detentions or similar has to snap shut on their tardy ways.
Make sure that your sanctions are sufficiently unpleasant that they want to avoid them in future, or else what is a sanction for? Sanctions, applied rigorously, consistently and fairly, should do the trick for all but the hardest pupils.
It sounds simple because it is. This is not a complicated problem and the solution must not overcomplicate the circumstance. Very good luck to you.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. http:behaviourguru.blogspot.com
Post your questions at www.tes.co.ukbehaviour.