Physically Unclonable Functions (PUFs) are potential security blocks to generate unique and more secure keys in low-cost cryptographic applications. Dynamic random-access memory (DRAM) has been proposed as one of the promising candidates for generating robust keys. Unfortunately, the existing techniques of generating device signatures from DRAM is very slow, destructive (destroy the current data), and disruptive to system operation. In this paper, we propose \textit{precharge} latency-based PUF (PreLatPUF) that exploits DRAM \textit{precharge} latency variations to generate signatures. The proposed PreLatPUF is fast, robust, least disruptive, and non-destructive. The silicon results from commercially available $DDR3$ chips from different manufacturers show that the proposed key generation technique is at least $ \sim 1,192X$ faster than the existing approaches, while reliably reproducing the key in extreme operating conditions.