A load and concurrent respondents check-routine in LimeSurvey, which allows you to stop people from entering the survey with certain load values or number of respondents in the survey. The counting of respondents should be primitive (e.g. calculated every 90 seconds, so that you might get 20 users in the survey, even if the routine has a limit of 5), but will still help to not get into the zone where everything breaks down.

Load limit ( depending on CPU available):
Respondents limit:
Message to show when entering of survey is blocked: Default text.
Something like "Too many respondents at the moment. Please come back later" or.
"Too many requests. Server is busy. Please come back later".

Limits might be survey-wise or installation-wise.