With freehold business sales you are obviously selling the freehold property but also dealing with issues like staff, stock, inventory, goodwill, contracts and this involves a lot more detail and therefore can add time to the process. Even with buyer and seller wanting the deal to exchange and complete as quickly as possible the legal process will usually take longer than expected. It is also important to remember that the owners of the business will usually still be involved in the day to day running of the business and they have to maintain focus on running the business as well until the sale completes.
With leasehold businesses there is an added layer of complexity in that the freeholder (the owner of the building in which the business operates) will have to approve the assignment of the existing lease or to grant a new one as part of the process of buying/selling the business. With private landlords it will depend on how well they communicate with their current tenant and their representatives as to how quickly the process can happen. In the case of pubs being sold on a leasehold basis where the freeholder is a dedicated pub company they will have their own processes, paperwork and time scales that need to be factored into the process.
Overall, it is advisable to expect the process to take at least 3 months to happen and in many cases longer than this. A business can be marketed for sale and a price agreed between buyer and seller very quickly however the formal, legal process takes much longer and ultimately must ensure that everyone is protected as part of the transaction.