Consider to upgrade your WordPress server for ZPress? This is definitely the right place to read through before performing the correct upgrades for your VM. There are a different factor to consider.
- Your are experiencing frequent slowness / lag when you're visiting your website at a very good internet traffic connection (and a reasonable good computer of course).
- You're expecting a predictable large traffic coming in a period of time (says a super large event for 2 weeks!)
- Your VM CPU performance's steady consumption (the flat line, not the spikes) is steadily at 50% and above.
- You can't upload anymore due to no free space (you used up the entire disk space).
- Serving a frequently crashed server will give a very bad impression for your visitors. Hence, if you're experiencing frequent crashes, you should upgrade.