How concerned do I need to be about DoS attack: ACK Scan listed in my admin log?

Netgear's firmware is great at creating false reports of DoS attacks. Many of them are no such thing.

Search - NETGEAR Communities – DoS attacks

Use Whois.net to see who is behind some of them and you may find that they are from places like Facebook, Google, even your ISP.

Here is a useful tool for that task:

IPNetInfo: Retrieve IP Address Information from WHOIS servers

The external sites in your list look safe.

If these events are slowing down your router, that may be because it is using up processor time as it writes the events to your logs. Anything that uses processor power – event logging, QoS management, traffic metering – may cause slowdowns. Disable logging of DoS attacks and see if that reduces the problem. This does not prevent the router from protecting you from the outside world.