As mentioned in other answers, It takes forever because the size of cocoapods master repo is huge. This time can be reduced using the following steps.
1) Create a private specs file path on your github repository. Provide this path https://github.com/yourpathForspecs.git' as a source in your podfile.
2) identify ALL the repositories You need and their dependencies( mentioned in the podspec.json file on cocoapods for these repositories) and get their podspec.json files from cocoapods. add these podspec.json files with their folder ( say the latest version folder for bolts) in this specs repository.
3) remove the source 'https://github.com/CocoaPods/Specs.git' in the podfile
4) pod update
This will take significantly less time as this requires fetching and downloading just the pods you need instead of whole cocoapods repository. In My case it reduced the pod update time from 15-20 mins on average to 3-4 mins at most.