This is not an easy question, since there are so many things to consider that I may miss some of them.
1- In case you are planning to use an ASA, check this to find the ideal throughput for you:
2- What type of traffic? TCP or UDP?
Make sure you avoid fragmentation, adjust the MSS value if necessary.
If fragmentation must happen, then set the DF bit to clear to allow it.
3- Make sure the clients always negotiate DTLS (which is by default).
As mentioned in the document above:
"Maximum throughput measured under ideal test conditions.
VPN throughput and sessions count depend on the ASA device configuration and VPN traffic patterns. These elements should be taken into consideration as part of your capacity planning."
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