to be taken until 1808. This essentially maintains counting slaves as 3/5 of a person until the way the census is calculated is changed, and this cannot be done before 1808.

The Constitution essentially makes provision to maintain the practice of slavery as it was at the time, and in that sense is neither pro-slavery nor anti-slavery. However, in mentioning slavery in the document, slaves are referred to as “all other persons” (in which case every person not a slave is filled into a previous category) or a person “held to service or labour.” The term “slave” is not used. This could be interpreted as an effort not to acknowledge slavery for what it was, or it could be just an attempt to use less offensive language.

The fact that Article 1, section 9 allows an import tax on people, relegates slaves to a commodity. I’m not sure that can be interpreted any other way. The fact that the Constitution prohibits changing the census process until 1808 could be seen as a safeguard to allow the country to settle into its new legislation before debating such a serious issue.

Thus there are arguments for calling the U.S. Constitution either “pro-slavery” or “anti-slavery,” and you’ll have to decide which is more convincing to you given the context of the writing of the document. I've included a link to the government archive page with the full transcript of the Constitution. The red text indicates portions that have been changed over the years, and links to more information about that.

Among the delegates to the Constitutional Convention of 1787, there were a number of radically different, and conflicting, positions regarding the issue of slavery. Some individuals believed that, having just successfully fought a war on the basis of liberty, it was difficult to reconcile that fundamental principle with the existence of slavery. Others, such as George Mason, argued eloquently that slavery would one day bring down divine wrath upon the United States:

"Every master of slaves is born a petty tyrant.  They bring the judgment of heaven on a country.  As nations cannot be rewarded or punished in the next world, they must be in this.  By an inevitable chain of causes and effects, providence punishes national sins by national calamities."

Southern delegates, not surprisingly, argued passionately for the continued existence of slavery. The vast majority of America's slave population was concentrated in the South, and the "peculiar institution" was the backbone of the Southern economy. For Southern delegates such as John Rutlegde, the question wasn't moral nor religious; it was a matter of private property rights.

There was no chance whatsoever that the Constitution would ever be ratified without the support of the Southern states. Inevitably, then, some compromise on the issue of slavery had to be found. The first component of this compromise was the notorious Enumeration Clause outlined in Article I Section II. For the purposes of taxation and political representation, it was necessary to calculate the population of each state. As a large percentage of the Southern states' population consisted of slaves, the question arose as to how they would be calculated.

The ensuing compromise treated each slave as 3/5 of a person. This ensured that the slave states would enjoy the best of both worlds. Not only would they be permitted to retain the institution of slavery, they would also receive more seats in Congress and more electoral votes than they would've done had slaves not been included in the population tally. The defenders of slavery had been provided with an effective veto against any attempts by the Federal government to abolish slavery.

But mindful of the possible expansion of the United States and of its potential dilution of their power, the Southern states insisted on a Constitutional guarantee against the abolition of slavery until at least 1808. There are two places in the Constitution where the compromise is set out. Article V, which dealt with the procedure for Constitutional amendments, expressly forbid any such amendment being made in relation to slavery.

And in Article I Section 9, Congress was prohibited from banning the importation of slaves before 1808. This compromise allowed not just the institution of slavery to continue, but also the slave trade. But as the slave trade was more widely opposed than the institution itself, the Article I Section 9 compromise effectively put those who traded in slaves on notice that the days of their business were numbered.

Finally, the Constitution addressed the thorny issue of fugitive slaves. To those in the Southern states, slaves were no more than items of property. And if their slaves escaped to free states, then their "property" should be returned. Once again, the slave states got their way, and Article IV Section II stated that slaves were still to be regarded as such, even if they managed to escape to a free state. This was a hugely controversial measure. The Southern states were especially vocal when it came to the issue of states' rights, yet here they were, blatantly undermining the rights of free states not to allow slavery on their territories. In effect, the Constitution was applying the laws of the slave states to the free states by the back door.