Informal, or formative assessments are about checking for understanding in an effective way in order to guide instruction. They are used during instruction rather than at the end of a unit or course of study. And if we use them correctly, and often, yes, there is a chance instruction will slow when we discover we need to re-teach or review material the students wholly "did not get" -- and that's okay. Because sometimes we have to slow down in order to go quickly.
Alber, Rebecca (Feb. 15, 2011). Why Formative Assessments Matter, Edutopia. Retireved from the web: https://www.edutopia.org/blog/formative-assessments-importance-of-rebecca-alber
One student commented, "I like having to take pretest and formatives because it gives me a hint of what the topic is about, and the formatives that we take to practice tests really helps me to improve what I don't understand."