Ribonuclease V1 (RNase V1) is a ribonuclease enzyme found in the venom of the Caspian cobra ("Naja oxiana"). It cleaves double-stranded RNA in a non-sequence-specific manner, usually requiring a substrate of at least six stacked nucleotides. Like many ribonucleases, the enzyme requires the presence of magnesium ions for activity.
Ribonuclease requires the presence of magnesium ions to slow its activity.
The context actually says that the magnesium ions are required for activity, not that it would slow them down.