It's hard to say what could be causing the lagging problems. Are other applications open and running in the background? Some apps, such as web browsers, can be memory hogs. Rebooting the computer into a fresh session can resolve some issues. Temporary files can build up in various locations (especially in Windows-based PCs). Cleaning up those files can boost performance.
The Illustrator AI files themselves can create their own problems, depending on their complexity and file size. A document raster effects setting that is set too high for certain types of layouts can cause problems. For instance, a vehicle wrap design set at full size but with a raster effects setting of 300ppi would create a serious burden for the computer. A lower setting of 72ppi would be more manageable.
The computer system's hardware is another issue. If a computer has only 8GB or 16GB of RAM it won't be able to handle complex Illustrator documents very well, much less handle multiple documents at a time. The quality of graphics board in the computer is also important. Drivers for various pieces of hardware, especially the graphics board, usually need to be kept up to date.