I think the CDS has just slowed down a lot today, after a speed decline over the last couple days, and maybe you’re seeing the same slow downloads I’m getting for the ERA5 single-level and on-pressure-level sets. And your switching to ERA5-Land and getting slower downloads is a coincidence because you did it at the same time that CDS got really slow.
Discussion and some metrics over here on this other post: Requests status always is Quened - #12 by Andrew_Janke.