Flow requires "object caching" such as memcache.
Extension:StructuredDiscussions#Dependencies
My host doesn't offer memcache, but does offer Pear:Cache, Pear:Cache_Lite, opcache, and apcu. Would any of those work?
Topic on Project:Support desk
According to my personal experience, APCu is OK, preferably memcached. Memcached installation is not so difficult.
my hosting doesn't support memcache
Use APCu.(Simply make sure its installed, then set $wgMainCacheType = CACHE_ACCEL , and it should all work. The MediaWiki installer will usually do this as the default if you have apcu installed.)
You only really need to move to memcached if you have multiple apache servers. If you only have a single apache server (almost certainly the case if you are on shared hosting like you describe), then there's not a whole lot of benefit of using memcached.