Pagefile.sys is the paging file for the Microsoft Windows operating system. The file is created and operated automatically by the system and has the sole function of lightening the load on the working memory when handling programs and processes that take up a lot of processing power. To this end, whenever there isn’t enough space in the available working memory for all data, Windows first expands the addressable system memory. The system can then assign the relevant files into this additional address space and shift them into pagefile.sys.
For applications, it makes no difference whether the files are cached in pagefile.sys or the physical memory. That said, access to the page file is significantly slower for technical reasons, which is why there can be substantial performance loss if large quantities of data are moved to pagefile.sys. Depending on capacity and the system version, the file can vary from a few megabytes to more than a gigabyte. For pagefile.sys in Windows 10 (on most devices), for example, the latter is the standard.