Shared ownership schemes almost always take a bit longer as there are more requirements, how much longer depends on the scheme. There have been so many different ones over the years so you have to read through from scratch each time. Some are almost like a second mortgage so don't add much if any extra time. The ones that involve staircasing or where a share of the property is being sold take longer. Much, much longer sometimes. The council ones seem to be the quickest.
Searches also vary massively. My local authority is taking three days at the moment. The next door local authority is taking about 25 days. The Vale of the White Horse estimated something ridiculous like 36 days, although it did actually come back quicker. Those are working days so once you add in weekends you are talking 7 weeks! It depends on the workload and staffing levels at each authority.
Is the valuation for the shared ownership scheme or the lender? Mortgages are taking ages to come through at the moment.
Leasehold properties usually take longer also. Often landlords/managing agents don't exactly rush to get the information to you and if you need to query anything it can be very difficult to get the information you need.
Please be wary of trusting developers' build dates, they aren't always accurate. Especially, as mentioned above, if you have very bad luck you could end up waiting for a road to be built before you can complete, even if the property is ready to move into.
I hate new builds. Some builders appear to think they are God. They try to impose unrealistic timetables, even if the property is far from complete. They also insist on clauses in contracts that no one else would get away with. Check your contract carefully and, if you exchange with completion to take place on notice make sure you insist on a long stop date.