One way to reduce the LFO rate for indexes larger than 25 would be to use the Capytalk test

(!Index lt: 25)

This would evaluate to 1 when !Index is less than 25 and to 0 when it is 25 or larger. Then you could use that as a scale factor on the LFO rate, for example

( |SoundToGlobalController| L * (1 + (!Index lt: 25)) + 0.85)

That would multiply the rate by (1 + 1) for indexes less than 25, and by (1 + 0) for indexes 25 or larger.