'Wisely and slow, they stumble that run fast', says Friar Laurence to Romeo. And he might just be right. The haste enters the play with the meeting of Romeo and Juliet (before that it's been a rather slow build to the Capulets party and a lot of exposition).
Romeo turns back from leaving the party in order to immediately find Juliet at her balcony (or her window... stage tradition is a powerful thing): before he leaves her later that morning, they have already vowed to be married, and, before lunchtime, Romeo has Friar Laurence agree to marry them the same day.
Yet what actually precipitates the tragedy itself is the death of Mercutio and Romeo's murder of Tybalt which follows it - and it's perhaps the impulsiveness of Romeo which causes him to murder Tybalt, as Benvolio says, before anyone could draw a sword to stop it. So the haste rushes the play onto its crux point (the lovers, are, of course, 'death-mark'd') but it's perhaps Romeo's personality that twists a comedy toward tragedy.
Though Friar Laurence's plan to save the day only fails because of a letter which Friar John is unable to deliver: if you read what Friar John says, he claims a plague stopped him from being able to inform Romeo that Juliet is not really dead. Romeo then kills himself because he thinks she is dead: and then, well, vice versa.
So in the end, is it a tragedy of haste? Not really. It's more a tragedy of fate - or, if you like, of bad luck.
You could certainly make a case for the fact that hasty decisions lead to the tragic events in "Romeo and Juliet". Everyone seems to be in a rush in this play, and the pace seems to pick up as the play unfolds - the proverbial snowball effect.
Romeo and Juliet fall in love quickly, decide to marry quickly, together with Friar Laurence concoct a plan to outwit their families quickly, and pay for this haste with their lives. There is a sense of urgency that these teenagers feel, and they are then able to convince others (Friar Laurence, the Nurse, etc.) to assist them in their hasty plan.
Haste comes into play in other places in the story as well. Lord Capulet's haste in promoting Juliet's marriage to Paris is what propels Juliet into such urgent behavior. Certainly Mercutio and Tybalt also suffer from hasty decisions in defending their families' honor.
Haste certainly does make waste in this tragedy.