I guess there are file consistency considerations that require vbox to faithfully carry out every disk write access required by the guest OS to the physical disk. File systems normally assume that disks perform write operations roughly in the order as they are issued (especially if the hard disk is explicitly instructed to flush its cache between writes). If vbox adopts any delayed writing or large write cache, the ordering may be violated and the file system in the guest may get corrupted if the vm is not closed properly.
Read caching, on the other hand, is really handled by the operating system and I believe it's pretty good in both Linux and recent Windows. The only thing you can do is to add more physical memory or close programs on the hose system when you're using the VM.
You may be able to reduce disk access due to paging in the guest. Simply allocate more memory to the VM and shrink/disable the page file.
(Edit: typo fixes)