PERF: Fix quantile perf regression
Closes https://github.com/pandas-dev/pandas/issues/35049

```pyhon
591 µs ± 29.5 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

440 µs ± 22.8 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
```
Should we take a more general look at where we can/should do string formatting more lazily? Interesting - so the evaluation of q / 100 makes things slower right? I would have thought generally f-strings would be more performant given things like this:

https://stackoverflow.com/questions/43123408/f-strings-in-python-3-6

But I suppose that assumes that the expression needs to be evaluated, which isn't the case here unless an exception occurs Thanks @TomAugspurger 