Update: We have since changed our sessions to table based sessions. On a running survey with a large number of participants, the server collapsed almost instantly (within two minutes). Our hosting provider identified the problem: session entries are deleted from lime_sessions based on the value in the column 'expire'. Since this column did not have an index, the request took longer and longer as the session table was growing, blocking the table for other requests. By creating an index on the column 'expire', the issue could be resolved.

I realize that this is only an issue when there is a huge amounts of entries in the session table. However: could it be an option to make it the standard way for LS to create the session table with an index on 'expire'?

Yes , sure , can you report the issue please :
community.limesurvey.org/bug-tracker/