I couldn't find a clear answer and I think a lot of people could have major advantages by knowing the answer to this..
So, last night when I was finished at work our development website worked like a charm. Nothing changed between then and this morning. But for some weird reason magento frontend now gives me a 503 error or a VERY long loadtime.
So I checked logs and such, and since I use redis cache and redis session cache I disabled those. I tried memcache and even the normal settings. Nothing seems to work.
Now how can I discover what is killing the website?
If this is a duplicate I'm sorry. Couldn't find a good answer.
EDIT: Forgot to mention, sometimes the homepage for example loads after a LONG load..
you should use some profiling tools to find where the app is slow.
In dev you could use blackfire or xdebug+kcachegrind
In prod blackfire or xhprof