Fast-unstake benchmarks are too slow

[ggwpez]

Benchsuite running since 14 hrs on ref hardware. Fast-unstake taking up the majority of that.
Looking at the quick-benchmarks Substrate CI shows that it is the on_idle_check which even with --steps 2 --repeat 1 takes 10 minutes:

09:18:00 Starting benchmark: pallet_fast_unstake::on_idle_check
09:18:06 Running Benchmark: pallet_fast_unstake.on_idle_check(1 args) 1/2 1/1
09:28:41 Running Benchmark: pallet_fast_unstake.on_idle_check(1 args) 2/2 1/1

Kusama does not suffer from this, so probably just parameters?

cc @kianenigma

[kianenigma]

will take a look soon.