NOEDIT: Will a software firewall (an antivirus firewall) be able to mitigate a potential attack? I know it'll block the connection but will your internet speed still take a hit?
For server crashing, yes. Your firewall will simply not allow the IP to connect to the Blockland server at all.
For Dosing, yes but probably from your router. Your router can block any incoming connections from a certain IP, but it may affect your speed, in which you will want to contact your ISP to block them on a lower level.
For DDosing, no. Your router cannot block all the malicious IPs at once. Your ISP may already detect that you are being DDosed, in which they might change your IP or do something else to prevent it from happening.
Dont trust me 100%, since I do not know everything about this field.