That’s normal, and likely not the reason you’re experiencing slowdown.
Any otherwise free memory, on Linux, will typically be dedicated for cache and buffer space by the kernel, until it’s otherwise needed.
This is normal operation for the Linux kernel.
For example: I have 32GB of RAM, 13GB of which, is currently being used for cache.
If I were to preform a lot of disk I/O, that number would likely jump up even higher.
The 723MB of swapped pages is mildly concerning to me, but can also occur during normal operation.
What does your drive I/O usage look like?
You may want to consider adjusting your systems “swappiness”.