Just for the record, the favicons database (webpageicons.db) *doesn't* grow endlessly anymore since 1.17. It's pruned at startup, and old unused entries are deleted.

Anyway, from this thread, i believe the partition is heavily fragmented, and any disk access is very slow. As a test, you could just unpack owb in ram (or another partition) and run it from there, as a comparison.

Finally, if you want to switch off most disk access, you need to disable favicons, history, form suggestions, session recording and so on. You can also enable private browsing to disable all these, since the primary goal of this option is to avoid keeping any trace of what was browsed.