If you do use CSF on a dedicated server (not a VM) then use the IPSET option, it will not slow down as you start blocking 10's of thousands to millions of IPs.
Using IPTables to blocks 1000s of IPs starts to delay your time to first byte.
High Performance, Low Contention, Hosting
Elastic Sites (HIgh Powered Hosting Accounts) with 1 to 20 CPU Cores, 2 to 40GB of RAM. (Power of a VPS without the hassle)
Shared, Reseller, VPS, Dedicated & More available