Air carrier Captain reported GPS interference procedures in a given airspace; need updating; as it exceed RNP limitation of INS and terrain escape procedures.

Narrative

GPS Interference time to unable RNP.Our GPS Interference and Spoofing procedures mention after GPS OFF that ground based navaids then inertial position likely will keep sufficient ANP for hours while slowly deteriorating. In every case I've experienced that is true except over the stretch of Lahore and Islamabad airspace between SULOM and MOTMO. Perhaps it's due to lack of ground based navaids in that terrain and political environment but along that stretch; in as little as one hour or slightly under; I've experienced UNABLE RNP as the ANP more quickly exceeds RNP here than in any other theater I fly and implement these procedures. Usually you see very little degradation but here it can go faster than I expected until I experienced it. As we learn on the fly; the statement of taking hours to see ANP exceed RNP is generally true but the possibility of significantly shorter times might be offered to operators as further information as to what might be expected. Perhaps adding to the more rapid loss of ANP than I expected is that we manually tuned terrain escape procedures but that is part of that section of flying so that's likely to remain a part of the mix. We were continually tuning appropriate navaids for terrain escape and were not letting irrelevant and distant navaids remain tuned after their use. Just more info to inform us of details of our new circumstances and perhaps worthy of dissemination.