Slow indeed:
It is estimated that, in the 17th and 18th centuries, 1.4 million slaves were compelled to make the trek through the Sahara [..] 1.2 million slaves are estimated to have been sent through the Sahara in the 19th century. In the 1830s, a period when slave trade flourished, Ghadames was handling 2,500 slaves a year. Even though the slave trade was officially abolished in Tripoli in 1853, in practice it continued until the 1890s. - https://en.wikipedia.org/wiki/Trans-Saharan_slave_trade
Robert Davis estimates that slave traders from Tunis, Algiers, and Tripoli enslaved 1 million to 1.25 million Europeans in North Africa, from the beginning of the 16th century to the middle of the 18th (these numbers do not include the European people who were enslaved by Morocco and by other raiders and traders of the Mediterranean Sea coast). - https://en.wikipedia.org/wiki/Barbary_slave_trade