I checked and how did you say that all plugins are disabled? I can see some still active like the "On site notification". The site size is not large enough to cause any issue to Q2A and 99% the issue is with some plugin - I cannot find any typical slow plugins though.
Also, voting is pretty fast when I checked. User register was very slow and asking question was also taking time. The other issue that can potentially happen is Q2A database might be missing some index - just do "Show processlist" in mysql terminal while posting a question and you should be able to catch any slow queries here.