Does anyone have any experiences getting the UK visitor visa

My family and I will be traveling to Paris next month on the 3rd. I just realized that we can take the Eurostar for a quick day trip to London, but the only problem is that my wife is not a U.S. citizen, she is Bolivian, so she would need a UK visa. Would it be worth it to try to get a visa now with only 3 weeks left? We did the application and she has an appointment to do the biometrics on May 14th but i've noticed different processing times from different people, we have until Monday to cancel the appointment if we decide not to go forward with it. We already have her Schengen visa in her passport, so im worried about cutting it too close and not getting the passport back in time to travel on the 3rd.

The UK website says 98% of people receive their visa within 15 days. Is this number something I can base my decision off of? Does anyone have any experiences applying and receiving the visa?