Seeking professional advice for resizing the cloud server spec

Question

I am using Droplet from Digital Ocean to host some of my web applications and am recently experiencing some traffic/loading issues.

Those web applications are all about sending data (inc images), writing data to DB, and fetching data from DB through API. (Frontend & Backend structure)

As the number of users grows, I think those processes are taking time which makes the slow loading issue (sometimes crash) I believe.

But I am not sure which one I need to choose in this case, Do I need to increase Memory? or CPU? I asked this question to Digital Ocean but didn't get a clear answer.

Current: 4 vCPUs, 8 GB Memory

CPU Optimzed: 8 vCPUs, 16 GB memory

Memory Optimized: 4 vCPUs, 32 GB memory

Thanks in advance for your recommendation