There is some difference, but it is marginal indeed at least in terms of speed. Via the traitional route with a real dummy, the display hardware needs to produce two sets of displayport output streams (one of which is converted to HDMI via a DisplayPort-HDMI controller chip MCDP2900 to drive the Dummy) + sync up the two displays in terms of vertical sync (this does not always work well, this is why real HDMI dummy users experience mouse jittery sometimes) and also has to scale the full-res framebuffer to two independent display (but scaling is done super efficiently on M1). With BetterDummy all this is not needed obviously.

For ppl who until now had to resort to mirroring the internal displays (as M1 MacBooks - before the new MBPs - support only the internal display + a single external display) the benefit is more obvious, they now can use clamshell mode and don't have to drive the full MacBook display hardware (with brightness turned down to zero) all the time.