Still need more data, as the other replies have said.
Think back to what you were doing when this slow-down started. A lot of times, it's related to some new app/utility/driver install. "Beachball"s indicate some core service is busy. Again, as was already mentioned, run Activity Monitor (in Utilities folder) and set "Show" to All Processes. Sort by "%CPU". Hopefully, this will point you to the offending process.
If it's not a process, then the system may be "thrashing" the HD when swapping memory out. If the hard drive is very full, the system might not have contiguous free space for swapfiles. There's a performance hit when a swapfile is fragmented.
One more thought: is it just Mail that gives you the beachball? Try rebuilding your Mailbox(s).
Think back to what you were doing when this slow-down started. A lot of times, it's related to some new app/utility/driver install. "Beachball"s indicate some core service is busy. Again, as was already mentioned, run Activity Monitor (in Utilities folder) and set "Show" to All Processes. Sort by "%CPU". Hopefully, this will point you to the offending process.
If it's not a process, then the system may be "thrashing" the HD when swapping memory out. If the hard drive is very full, the system might not have contiguous free space for swapfiles. There's a performance hit when a swapfile is fragmented.
One more thought: is it just Mail that gives you the beachball? Try rebuilding your Mailbox(s).