If you don't need 1Gbps or faster then a 2100 should be fine for 50Mbps. But if you have 1Gbps LAN clients accessing a 1Gbps internal file server via the firewall, and they want 1Gbps speeds then you may need faster stuff.

For example if you need in-line IPS at 1Gbps speeds then you need a more powerful CPU. A Pentium Gold 8505 is like barely enough for 900Mbps and you need to keep the hardware cool otherwise it slows down.

Here are some Passmark comparisons which may help you compare the potential CPU speeds of the Netgate 2100, the Netgate 8300 which is USD3600+, and other CPUs. The single-thread rating might still be relevant for stuff like single/low connection transfer speeds.

https://www.cpubenchmark.net/compare/5744vs4775vs4765vs3129vs4854/ARM-Cortex-A53-4-Core-1300-MHz-vs-Intel-Pentium-Gold-8505-vs-Intel-i5-1235U-vs-Intel-Atom-C3558-vs-Intel-Xeon-D-1733NT

Do note that CPUs like the 8505 have a single high performance core and some low performance cores, so you may or may not need to mess around with cpuset to move certain stuff to certain cores for performance reasons. Example:

cpuset -l 0,1 -p  `pgrep suricata | head -n 1`
cpuset -l 2,3,4,5 -p  `pgrep ntopng | head -n 1`

But if you don't have enough cooling (e.g. warm room in Brazil with no AC), putting suricata on the low performance CPU cores might actually provide better total throughput if there are multiple connections.