Change admob mediation default timeout

I’m using admob mediation to mediate between ad networks in my game. I’m still using old admob btw.
For interstitial ads, the first network in the order is admob interstitial. However, the fillrate is only 70-80% which i read should be higher for admob interstitial (or i might be wrong). I suspect this is because of the default timeout for admob mediation, when the internet connection is not good enough, the ad request timeout. The question is, how could i change the default timeout for admob mediation? I cannot find the setting in the old admob website nor via code. Should i upgrade to new admob? Thanks.

any way you have to have upgrade to new admob before august