If Fundsmith's strategy still works, you buy more when it underperforms – that's how investing works. It's largely underperforming because Nvidia's carrying markets – and that's just a random thing that's happening. Strategies can go through 15 years of underperformance – like value investing.
If it wasn't for the fee, it would always be as good as 50:50 whether Fundsmith outperformed over any period... But the two problems are: you don't know whether the strategy still works (so you're just performance chasing), and there's a high fee for a fund that doesn't do much..
So if you can't solve these problems, and you are just performance chasing, then as soon as relative momentum turns against the fund (vs the index) you sell the fund and switch into the index. There's nothing else you can do that makes any sense. Other than just buy the index in the first place, because you simply don't know whether a strategy's good or not. You're just going on past performance, which is largely random.