Learning analytics (LA) dashboards are becoming increasingly available in various learning settings. However, teachers may face challenges in understanding and interpreting the data visualisations presented on those dashboards. In response to this, some LA researchers are incorporating visual cueing techniques, like data storytelling (DS), into LA dashboard design to reduce the data visualisation skills -- often referred to as visualisation literacy (VL) -- and cognitive effort required by teachers to effectively use dashboards. However, despite the potential of DS principles in simplifying data visualisations, there is limited evidence supporting their effectiveness in actually reducing teachers' cognitive load. The study presented in this paper addresses this gap by investigating the potential impact of LA dashboards, with and without DS elements, on teachers with varying VL levels. Through a quasi-experimental study involving 23 teachers, we analysed changes in pupil dilation -- a proxy for cognitive load -- as they examined LA dashboards featuring student data captured while participating in synchronous, online collaborative learning tasks. Our findings suggest DS can reduce cognitive load, particularly for teachers with lower VL. These results provide insight into the effects of DS and VL on teachers' cognitive load, thereby informing the design of LA dashboards.