Will adding a software firewall with bfd/antidos and snort slow down the access times to a server as opposed to a server without any filtering in place? I'm afraid that too many rules and filtering may slow down access times.
In most cases, enabling a firewall will not slow down the access times by a noticeable amount. It takes very little processing power to process a rule, and even with hundreds of rules, it would be done well before the next packet shows up. In fact, many people are still using 486s and low-end Pentiums as firewalls.
I can vouch at least for APF(Advanced Policy Firewall) that you wont notice any difference. We are pushing 800 rules and based on benchmarks, they really hasn't been a difference that I could say would be legit, taking into account network/server fluctuations. The benches was done on a Pentium 3.2 HT, I can't speak for a little celly