As one of the most frequently occurring operations, I/Os significantly affect the application launching time and frame rate of mobile devices, hence influencing the user experience. However, the response speed of I/O requests is still the bottleneck in practice. This paper shows that high I/O latency is usually due to the congestion inside Flash instead of the system layer. Unfortunately, Flash is treated as a black box device and cannot be modified after delivery. In this paper, we propose a novel service to address this issue without an intra-Flash modification. Specifically, this paper proposes a lightweight I/O throttling framework in mobile systems named FlashDAM. This service throttles the I/O flow to make way for I/Os that may block the foreground application. FlashDAM is the first work that proves that proper I/O throttling positively affects the user experience, contrary to the common belief. Furthermore, this paper proposes FlashDAM$^+$, an enhanced version of FlashDAM. By coordinating I/O throttling and compression, FlashDAM's effect in the system layer is minimized. We have implemented FlashDAM on real mobile devices. Experimental results illustrate that the app launching speed and frame rate are enhanced by 72% and 45% separately compared to the state-of-the-art. When enabling the compression feature of FlashDAM, that is, FlashDAM$^+$, screen jank and application launch latency are further reduced by 9.5% and 11.4%, respectively, under heavy background I/O load.