## 2 Results
### Determination of \(\Delta t_{\rm res}\)  
The central 20-fs of the WVD (between 52 and 72 fs) were split into 20 bins of 1-fs duration, each with its mean IF and \(\rm FWHM_{\rm in}\). For each bin, \(\rm BW_{\rm req}\) was calculated at resolutions between 0.9 and 10 A and for effective mosaic spread values between 0.05-0.5\({}^{\circ}\) using Equation 1 (Figure 4a). As mentioned above, the effective mosaic spread is a sum of the beam convergence and mosaicity contributions: at XFEL facilities, beam convergence at around 9 keV ranges from a few microradians (on the order of \(10^{-4}\) degrees) [51; 52; 53] for the unfocused beam, to tens of microradians if KB mirrors (e.g., \(\approx 47\)\(\rm\upmu rad\approx 0.002^{\circ}\)[54]) or compound refractive lenses (e.g., \(\approx 91\)\(\rm\upmu rad\approx 0.005^{\circ}\)[55]) are used for focusing. As this is considerably smaller than the mosaic spread of protein crystals, the latter is the determining factor. The range of mosaic spread used here considers very low mosaicity values, such as those from crystals of the photoactive yellow protein (PYP, \(\eta\approx 0.04^{\circ}\)[56]), as well as values up to ten times larger. From the \(\rm BW_{\rm req}\) of a reflection, it is also possible to calculate the amount of time it is stimulated for (Figure 4b). The \(\rm\Delta t_{\rm IF}/\Delta t\) for the chirp is 5.6 eV/fs, the required \(\rm\Delta t_{\rm IF}\) is given by \(\rm BW_{\rm req}\), and \(\Delta t\) is solved for. At mosaicity = 0.5\({}^{\circ}\), low-resolution reflections are expected to spend up to 8 fs in reflecting condition. On the other hand, high-resolution reflections at low mosaicity values will be fully \"instantaneously\" integrated by \(\rm BW_{\rm in}\) (Figure 4b,c). Figure 5 shows the predicted integration for a low- and a high-resolution reflection during the same interval in the pulse along with the linear chirp in IF. The 1.5 A reflection is fully integrated in 500 as, while the 5 A one is calculated to remain in reflecting condition for 2 fs. These time durations are dictated by \(\rm BW_{\rm req}\) for the individual reflections and by the  
Figure 3: Central slicing of the pulse with 4% chirp. **(a–c)** respectively plot the \(\rm FWHM_{\rm in}\), flux, and IF values when the pulse is sliced at these central 20-fs. The flux is constant across this central slice and the energy chirp is reduced to \(\approx\) 1.5%. The \(\rm FWHM_{\rm in}\) fluctuates between 40–10 eV. **(d)** Short-range “single-shot” autocorrelation of the central 20-fs of the pulse: the SASE spike width is on the order of 200 as. mean BW\({}_{\text{in}}\) in the time bin. Time durations were calculated from BW\({}_{\text{in}}\) values through time-bandwidth products. The instantaneous durations given by the mean FWHM\({}_{\text{in}}\) of each time bin specify the formal limit to the time resolution and vary between 50 and 150 as (Figure 6a). These values were calculated using a time-bandwidth product of 0.44 and therefore underestimate the pulse duration as the chirp of the single spike is not included in their calculation: in the presence of spectral phase, the spike duration is correspondingly longer [48]. Figure 4c shows the ratios between BW\({}_{\text{req}}\) and the available BW\({}_{\text{in}}\) given by the WVD. As expected, ratios decrease as a function of resolution, with values all below 0.4 at resolutions lower than 4 A. From these ratios, the number of energy points per reflection (that is, how many BW\({}_{\text{in}}\) are needed to integrate it) was derived (Figure 6b). Realistically, many of the energy contributions will overlap on the diffraction pattern, so the number of points per reflection was only calculated for bins with BW\({}_{\text{in}}/\text{BW}_{\text{req}}\geq 0.2\), setting a limit of 5 energy/time points per reflection.