You can get away with a single CPU and 1GB of RAM if your mail server is not too busy, but mail servers like MS Exchange require a lot of resources so increasing the CPU count to 2 or more with 4GB of RAM is desirable.Enabling SPF increases the work load (if using an external DNS server) so an additional CPU is desirable.
Ultimately a recommended system is a quad core CPU with 4GB Ram.
If your mail server is under a heavy load, MWES will throttle the connection.
MWES is set by default to handle 5 emails per second and this suits most scenarios, especially where more and more virtual environments are used where cpu's are being shared and clock cycles time sliced. If you have a dedicated server and a high work load, this parameter (max_smtp_channel=5) can be changed to a higher value in the Windows registry or Linux conf file. See here: Change settings outside the admin panel
If you do occasionally receive more than 5 emails per second, then MWES will throttle the connection until the backlog is cleared, so those extra emails may be delayed by a few seconds.