Slow loading sites

I have some trouble with the loading speed of two of my sites.
Both sites have been recently upgraded from zen-cart 1.3.8 to 1.5.4 (I started with this about two months ago). I used the official upgrade procedure, but didn't have any experience with zen-cart before this. There may be something I have overlooked because of this.
The sites run on PHP 5.4.36, MySQL 5.5.41 and both are on a debian linux server.
They can be found at www. carmo .nl and www4. carmo .nl
The first one is a live site, the second one is still a work in progress where I'm looking into using a responsive template.

For both sites I have done most of the points on the list found at http://www.zen-cart.com/content.php?63-how-do-i-speed-up-my-site. I didn't do 4b because I believe that isn't used at these sites, I tried 8 but that didn't work and turned that one back to 0, 13 is at the moment a bit to hard to understand.
As far as I can tell, the images which are used are not too large.
Running the optimize database from the admin interface on the www4 site hasn't helped either for that one. Haven't tried doing that on the www site.

There are several modules installed on both sites:

• ez pages multi language
• vat mod
• dual pricing
• ck editor
• several language packs
• ideal SEPA
• google merchant center
  The databases have been converted to utf-8

At the moment I have no idea any more of where to look to get the sites load faster. The live site has mostly trouble loading the homepage, the test site has trouble with all the pages as far as I can tell.

Does anyone has an idea of where I can look to speed up my sites?

The live site seems plenty fast to me. The other one is a little slower. My experience here tells me that slow loading Zencart sites are almost always caused by server problems. Are both sites on the same physical server?

You might try activating Admin - Configuration - Logging - Display The Page Parse Time and compare the results with this: http://www.zen-cart.com/content.php?145-what-do-the-page-parse-times-and-query-counts-numbers-mean

The sites are not on the same server, they both have their own server, no other sites are on those servers. The servers are at our own place, not the work location I am at but still close by, with a high speed internet connection.

On the live site, the parse time won't display. The number of queries and query time does.
Parse Time: - Number of Queries: 30 - Query Time: 0.016179927261353

The test site does display all the info, but the parse time varies wildly from 0.496 to 5.651
The number of queries is a lot higher (about 2745, changed several times), this is probably because right now I have the categories from one of the main categories displayed. Turning this of might help in the parse time, but that might only have a difference on the homepage when the rest of the site is also slow for me.
The query time varied somewhat between 0.19837912785339 and 0.28577972631836 in the time I tried this.

When I have loaded the sites several times in the same browser, the time before I can see anything is quite fast, but when I try loading even the 'fast' site, the live site, it takes can a long time, just a couple of minutes ago it took about 10 seconds. I don't think this is because of the internet speed here at work which is also high speed.

Both the servers are new, the first one is just over two months old, the other one even newer. There are no other sites running on those servers. Only the programs necessary for zen-cart (php, mysql, openssl,...) have been installed. I don't understand enough at the moment of servers on how there could be anything on here which might slow the site down so much.

I know this is a bit later, but I found something which made the site slower for some people, but not all. Maybe this can help someone else with the same problem, or maybe someone can tell me what causes this to happen at all on our new servers?

In admin -> Configuration -> Sessions, there is a line which says IP to Host Conversion Status. This was on true for all the sites, both the slow and the fast sites. Changing this to false on the slow sites has made the site just as fast as our other sites were.

I haven't yet have had a chance to look into why I have to turn this off on the sites on our new servers, but it has made a big change. There are no longer any customers complaining of a slow site, at work we don't have to wait long for the site to load and there have been more orders since turning this off.

Well, as identified in the sessions settings site slowdown is a possible affect of having it enabled. Basically additional processing/checking is needed before allowing the site to move forward, so yes if the host requires more time to accomplish the task then it wll slow things down. If on the other hand it is turned off then will process faster. I can't say though what "capability" is lost by turning it off per se...

Images could also be optimized to reduce the page load