It may be considered common knowledge that frequent root changes, i.e., a fast harmonic rhythm, usually cause a piece to be perceived as being more purely "harmonic" and less polyphonic in nature. (See for example, Piston's Counterpoint, p. 63.) Yet, many allegro-tempo instrumental pieces of the late-Baroque period that were written in the fugal style feature extremely melodically active basso continuo lines, implying a fast harmonic rhythm, the thing just mentioned as being "anti-contrapuntal". (Even if one takes into account the fact that in such pieces usually not all bass notes are harmonized.) A brilliant example is the third movement from Bach's fourth Brandenburg concerto, in which the continuo occasionally appears as one of the fugal voices.
How did composers of the era solve this apparent paradox? Were the root changes implied in their continuo lines in a sense illusory? Or did they employ special tricks and devies to make their fugal music sound properly polyphonic despite the fast harmonic rhythm?