Computer lag that occurs when loading programs, working with large files or loading new areas in games is often caused by the time it takes to move data to and from the hard drive. While you can't eliminate loading time entirely, incorporating the recommended amount of RAM for the programs you use cuts down on hard drive transfers and makes your computer work faster. If you already have enough RAM, however, you can't further improve the system's speed by adding more.
How RAM Improves Performance
RAM exists because hard drives are slow. Even in ideal conditions, the speed of the interface that carries data from a hard drive caps at about 600 megabytes per second -- and in real-world use, drives run significantly below this limit. Meanwhile, RAM can access data over ten times as fast -- up to 6,400MBps with the "Double Data Rate type 3" DDR3 RAM common in PCs as of 2014. By storing recently used data in RAM, computers don't need to wait for the hard drive between every operation, vastly improving overall speed.
More Isn't Always Better
Upgrading a computer's processor increases its speed, no matter if you use a computer for basic tasks or intensive processing. Additional RAM, on the other hand, does not improve speed directly. Your computer will slow down noticeably any time it runs low on RAM, because this forces it to read data directly from the hard drive. But if you have enough RAM for the programs you use to never run out, adding more won't provide an additional benefit.
How Much RAM You Need
The amount of RAM necessary so that you never run out varies depending on the programs you use. For example, while computer games can require a few gigabytes of RAM, they also run one at a time, avoiding the additional RAM requirements of multitasking. As of 2014, most computers used for gaming have 8GB of total RAM, with 4GB coming in second. Photo editing programs also have notably high RAM requirements: Photoshop CC requires a minimum of 1GB available RAM just to run, and the larger the photos you edit, the more RAM the program will use. Note that 32-bit operating systems, such as most versions of Windows XP and Vista, can't benefit from over 4GB of RAM. Most newer computers have 64-bit systems, which have no practical limit on RAM.
Checking Your RAM Use
To find out your personal RAM usage, run the programs you use on a daily basis and load a file or two in each to simulate regular use. Press "Control-Shift-Esc" to run the Windows Task Manager and open the "Performance" tab. Check the memory information -- if you have less than 1GB RAM available with your programs running, adding more could improve performance.
RAM Types and Speeds
Aside from the amount of RAM in a computer, systems have varying types of RAM, such as "Double Data Rate type 2" versus "type 3" -- commonly called DDR2 and DDR3. Newer types work faster, but you can't change to a newer type without replacing your computer's entire motherboard. RAM also comes in different speeds, such as "1333" and "1600," but these also need to match the motherboard to avoid bottlenecks. To prevent incompatibilities when upgrading, it's generally best to add additional RAM of the same type you already have.