Response time and ping will not decrease at all unless one of them is using the bandwidth heavily (file transfers typically). Without any QoS, one of the two could consume all the bandwidth and degrade network performance for both. However, during normal usage there should be no noticable difference.
If QoS becomes necessary, a cheap router with DD-WRT or Tomato installed would be an easy fix.
EDIT: Here are some possible scenarios the OP has asked for:
- Accessing different websites simultaneously: should see no impact.
- POP3 and downloading mails at the same time: little impact unless the mails are huge. Both clients downloading 20x 40mb mails - yes, they'll both go slower. Both clients downloading 20x 20kb mails - no.
- Donloading a file + surfing - yes, unless an FTP program with rate limiting is used to prevent the file download from using all bandwidth. OR, QoS is used at the router. This is where QOS would be valuable, because you could prioritize large downloads below web surfing and guarantee the surfer is not impacted.
- Bittorrent + surfing - same as file downloading, except all Bittorrent clients have rate limiters built in.
Bottom line is if you want both parties to do things without being aware of the other, and doing "polite" things like limiting their rates, use QoS.