AE creates cache files, on your harddisk, to store rendered frames which were previously stored in RAM when you create a preview. Cache Files are temporary and AE will delete them based on a FIFO (first in, first out) method when new frames are cached and there is insufficient space in the cache area to store all required frames, the previous cache files and the newly created ones.
When a cache area is totally filled, AE will noticeably slow down as it now has to delete previous cached files while also writing newly created cache files. Hence, you will want to ensure you never have a fully filled cache area. This is off course a difficult task but when you notice AE slowing down while creating Previews, then there is a high probablity that your cache area is filled and you will achieve better performance if you stopped the Preview, purged all cache and memory and re-start the Preview.