Can someone tell me how to configure ModSec to reduce resource consumption?

Unfortunately there is no any chance to configure ModSecurity to reduce resource consumption. What I can say is that 2 core (you didn't mention the types of core) and the 2GB of RAM seem very low for 100,000 requests (per second, I assume). That's lot of transactions, and each transaction uses lot of memory.

You can tune your instance (that the blog post mentions too) that which types of files should be inspected, then you can decrease the resources usage, but I'm afraid that's only a partial solution. Create a filter for this based on the file suffix is very risky IMHO, so be careful with that.