Yes. This Cvar command will turn Lumen GI on or off.
1# r.Lumen.DiffuseIndirect.HalfRes=1
2# r.Lumen.ScreenProbeGather.ScreenSpaceBentNormal=0
3# r.Lumen.DiffuseIndirect.SSAO=1
4# r.Lumen.DiffuseIndirect.TemporalAccumulation=0 (May show more noise and flickering)
Or/and r.DynamicGlobalIlluminationMethod=2
Test these and see if that can help to get better performance without turning GI off completely. But to just turn it off then use r.Lumen.DiffuseIndirect.Allow=0