Hello!
I just visited your website, browsed some pages etc. It seems that it is very slow only on the first access of the homepage (or any other page), then, on subsequent requests, it loads nicely, some 2-3 seconds. However, after a while (eg. some 30 seconds) of inactivity and reload of any page, it's again very slow, goes to 10-12 seconds. It could be a server overload (if you have root access to the server or WHM panel you can take a look at the output of the "top" command and try to figure it out), or there might be a caching plugin not properly configure to cache for a longer amount of time (30min - 4h) that times out and starts rebuilding some cache too shortly. It could also be some IP tracking plugin that does some queries to the db on the first access from a new IP. Most likely, in my opinion, it seems like server overload, because the lag is different across the requests, it now loads very fast, then very slow, then again, randomly, very fast and very slow ... this is, in most of the cases, caused by a lock on some of the mysql tables achieved by some poor query, that locks a table for write (insert or update), while the other threads/requests are waiting for seconds until some index is being rebuilt and the write operation is done. It's a very common situation, particularly on innodb tables (and I know that some WP version come in that flavour).
Regards,
Dragos