absolutely this is.
in November i slapped together a very basic wordpress site, 1.2 million posts and a few weeks later the site got insanely slow (even with it being on a 12core/64gb ram dedicated server, behind cloudflare and local nginx proxy, memcached/w3tc wordpress plugin nginx, php5-fpm, and apache solr search).
google page speed insights eventually scored me at like 40% because it was taking a few seconds for each page to load, if i recall correctly my TTFB (time to first byte) for page loads was something like 8 seconds, and it definitely wasn't a server/host/uplink issue. at roughly this time google all but stopped crawling my site, you can see in this image taken from my google webmaster tools as the longer it took for googlebot to crawl my site the less googlebot came back to crawl..
rather than slaving away with shitty wordpress caching modules written by absolute idiots i simply dumped wordpress and coded the site from scratch. fast forward a month to the present and the end result is google page speed insight's score is consistently 98% (I lose 2 points only b/c of external resource caching that is out of my control). now that i've scripted my own sitemap generator im seeing google indexing ~ 250,000 posts per day.
i share this story not to slam wordpress and the idiots that make it's nearly useless plugins, but rather b/c i realize that my site is definitely an extreme case (I post ~2 posts per second 24/7/365). if you think about it from google's perspective, they've got MANY websites for their bots to crawl, and if you website is gonna take more than 0.30 seconds to load then googlebot is gonna just drop it's priority to crawl/scrape your site for indexing.
if you haven't used page speed insights before you should check it out:
https://developers.google.com/speed/pagespeed/insights
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Added after 42 minutes:
also, forgot to add: if you want a really good tool for narrowing down what is bottlenecking your site's page load speed, you should sign up for newrelic. just dont give them your real name and don't pay for it.
one of the idiot sales guys @ newrelic freely gave out one of my competitors' real name, fortunately for him (my competitor) in this case hes a friend, but still after seeing newrelic's sales staff do something as stupid as that i told them to drop dead, so i basically used their tool for a month as i developed my site and got out there just in time