What you are describing isn't default behaviour.
Typically as FPC is primed, when not logged in - or having done anything to make the session specific to you, the page load timeline tends to be:
--1.2s--------0.8s------------------0.6s-----------------------0.1s---------------
Uncached Mage default cache Partially cached in FPC Fully cached in FPC
What should happen is that a single page load primes the Magento cache and to a lesser degree, the Magento FPC cache. The subsequent load for the same content fully primes the FPC cache resulting in a 3rd page load for the same content to take around 0.1s.
But when you do something to make your session unique, eg, add-to-cart - then FPC exhibits slightly different behaviour and the render times do increase slightly.
--1.4s--------0.8s------------------0.6s---------------
Uncached Mage default cache Fully cached in FPC
It sounds like you've broken the FPC logic by make your amendments (hole-punches) by essentially causing FPC to behave like in my second example.
Ie. It doesn't serve any content from a global cache, but rather only content specific to your session
My suggestion would be to either install a clean EE store - and watch the behaviour there - or move the local/community directories and custom theme package directories (to force enterprise/default to load).
There are two difficult things in computer science:
- Naming things
- Cache invalidation.
Hole punching falls into category #2 :)
General
The best approach is to start at the lower points of the stack and optimize up to the frontend of Magento.
Database and Filesystem
Should always be the first areas to focus on. Because. I/O.
MyTop is a handy Linux based perl script that will mimic the Linux 'top' command and give you insight on the state of your MySQL instance(s).
Htop is a more robust top, The strace feature can help determine ins/outs of a process to find potential bottlenecks.
Iotop is another tool to consider for monitoring I/O.
Other handy utility scripts like mysqltuner.pl and mysql tunning primer can offer insight into your MySQL runtime variables and offer advice to help. Keep in mind these are meant to be guides as the best approach is always an evaluation of requirements and tuning based on known data gathered. Blindly doing so can cause more damage at times than good. And prematurely running these without at least 24 hours of mysql runtime variables may offer bad advice.
Keep in mind Percona, MariaDB and standard MySQL should work with all of the above. Favoring Percona as a MySQL fork, since Magento is so heavy on InnoDB and XtraDB offers many tools and enhancements to the db engine.
Apache or Nginx
Still using Apache as it has served many others well, myself included. I have used and configured Nginx as well. While it does offer some advantages there is a learning curve. While the two are both popular options, it does offer some advantages over Apache, one would be a smaller memory footprint. However a slim downed Apache running PHP-FPM will have a similar memory footprint.
Case in point:
Since this article was about performance, I should point out that one
of the easiest ways to help apache get out of its own way is to not
use .htaccess files. Put what you'd put there in your Directory
stanzas, set AllowOverride to "None" and you end up not asking apache
to traverse the whole document path to figure out if it needs to pay
attention to .htaccess or not. This is a basic, simple tuning hint
that many people seem to miss.
To help facilitate this check out:
Utilizing a CDN to help take the ease off of either will help obviously but will have added benefit on frontend optimization since most end users browsers will be able to connect to both servers with the same number of connection limits. This also frees up Apache from not having to jump through checks and such just to serve up a simple static image. Lighthttpd is an option if you want to run a static web server just for content besides a CDN.
PHP
PHP-FPM and APC. Use them, strip out any unneeded or unrequired PHP modules not needed for Magento.
Magento codebase
AOE_TemplateHints is great to determine if your blocks are caching properly:
AOE_Profiler is good for profiling, be sure and enable its DB layer profiling (in a local/dev environment obviously). This in conjunction with the mytop tool mentioned previously makes finding bad behaving SQL an easier task.
3rd Party modules & Custom code
Some very good best practices for optimization from Magento themselves is a good read, and to keep in mind when reviewing 3rd party modules before using them. (there are lots of bad behaving ones IMO).
A tool Magniffer from Magento ECG will help easily identify bad behaving code based on the PDF provided above. It is symfony/php-parser based however but installable via composer.
Varnish
As an advocate of Varnish being the author was a FreeBSD kernel dev, it offers some crazy sub second load times. However if you even have some of the slightest differences in your templates that isn't out of box, you will spend time configuring varnish / magento to holepunch the content you need. Most I've seen will simply AJAX'ify the needed items uncached from Varnish.
There are a number of Magento modules to help facilitate this hole punching and caching:
Ultimately this should be at the last end of your optimization journey, and MAY require some customization to get things right.
Magento CE FPC
So far the best CE FPC I have found is: Lesti::FPC
it is a very well put together (all observer based) open-source and free FPC for Community.
At the end of the day use your own testing and judgement.
Some further reading:
Best Answer
My company develops Evolved Caching which is a FPC solution so I'm not a store owner but I will make a few comments on your question.
Firstly only you can really make a decision on which FPC solution is best for your store - I'm not going to say our solution is the 'best' as it means very little, everyone who develops FPC solution is probably going to say that. I would however recommend you trial all the options you can for free before committing to one, that way you can test the extension and the support before you buy (you can trial ours here if you are interested). In my opinion if a company is confident in their product they have no reason to not offer a free trial.
Secondly Magento has such wide possibility for development it's completely impossible for any solution to work flawlessly on all stores. If any FPC developer says their solution will work without fail on every install - just don't believe them as it won't be true. The key here is the versatility in the way the extension has been developed and the willingness of the developers to help you make the extension work correctly on your particular install. If they are unwilling to help, then you probably want to think about whether you want to go with them or not. Essentially if the extension has been developed well, and the store has been developed well, it shouldn't be too hard to get caching working correctly even if there are a few initial issues.
Thirdly if adding SSL has increased page load times by nearly 2 seconds - you definitely have a problem there. The only overhead SSL adds is the handshake and this shouldn't take anything like 2 seconds to complete - it should be adding milliseconds not seconds onto load times.
It sounds like you have pretty decent page load times already assuming you can get SSL sorted out and the 2.5 seconds is uncached load times so you are in a good position to consider a FPC solution, but as I said above, look at what available, test what you can and make your own mind up. Obviously I hope you choose ours, but if you don't that's ok ;)