It's been a good few years since I last paid in a lump sum – I decided the weekly option was better overall!
At the time you were only allowed to put in lump sums towards the end of the tax year, and I always started the process around January/February by asking for a quote as to how much I could invest. Bearing in mind you could only pay 15% of your earnings into a workplace pension at the time(pre 2006).
But I would agree that in 2019, these things really shouldn't take as long as they do. Especially when other financial transactions can be done electronically in a much shorter time.
Good point about lump sum payments stopping soon – it was mentioned in the AVC booklet that was sent out recently!