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Am I right in thinking that having increased the acceptable length for meta descriptions Google have now reverted to the old length? Yoast is now saying that meta descriptions that were OK are now too long.

There are definitely other optimizations you can do, and Visual Composer may not be the biggest slowdown. I just noticed when I viewed the source code that it was inserting a lot of separate CSS files and so it may be contributing some. Other optimizations like going through PageSpeed Insights' recommendations may get you farther. But any themes and plugins you use on a WP site, those resources are loaded across the whole site - even on pages they're not used on. So in a lot of cases, selecting a premade theme or having one built from scratch can make your site a lot faster than using page builders. Basically it's a tradeoff: do you want to pay someone to code a theme specifically for you and do the heavy lifting up front (or make design compromises and use a well-coded prebuilt theme), so the code that renders to visitors is lightweight; or do you want to just go wysiwyg on the admin side, so the code that renders to visitors is bogged down but you don't have to hire a developer?