This discussion regarding stylus drag is the reason why I sold some highly regarded belt drive turntables. I could hear them slow down during heavily modulated passages.
I didn't understand what was happening until i listened to a turntable that had the drive and speed accuracy to combat the stylus drag problem.
The turntables were a Garrard 301 and Technics sp 10 Mk III; but only after they were reengineered with heavy plinths or motor controllers or both to improve upon their weaknesses. The net effect is they powered through the record with dynamics and speed accuracy much improved over my former belt drive tables.
I drew the conclusion that all belt drive tables are compromised and cannot be speed accurate or very dynamic.
So in essence the stylus drag problem forced me to travel back to reexamine the old argument of which drive system is the most speed accurate and dynamic.
Not until I recently reacquainted myself with a belt drive turntable, the Micro Seiki RX 5000, would I even look at a belt drive design. I was certain you used a DD or an Idler so long as they were in a well engineered plinth with a good speed control device. But my recent Micro Seiki experience showend me it was dynamic as hell and with the surgical string speed stable.
I was very surprised and forced to once again admit there are no absolutes and be very careful before drawing any conclusions