I've recently gone through issue with IS installation (IS 4.2 SP 5) which took longer time to complete than expected (Even though all the prerequisites were in place). After some basic research , I found that the culprit is the mapped network drivers in the system. The workaround for this issue is covered in the IS release note.
Here are the details
Issue ID :
When you select Disk Cost in the Select features step of the installation, the installer calculates cost using all drives, including mapped drives. This may result in additional installation time if, for example, the network is slow.
Unmap the network drives before installing the software.