If this is purely a proof of concept, I'd go with WordPress. So much of what you'd need in an app platform is already there, plugins for anything & everything are plentiful, and it's incredibly easy to add any necessary custom functionality in via PHP. Depending on what you are building, you might not even need to build from scratch if it works.

Plugins like Advanced Custom Fields Pro, Gravity Forms, Custom Post Type UI, WooCommerce, and MemberPress might come in handy.

By far the biggest thing I've learned when it comes to my own productivity: find a way to block out social media and other distracting websites for set periods of time. There are quite a few good blocking apps out there if you do some searching. My personal favorite is Focus (Mac app) - https://heyfocus.com I have zero affiliation with them, I just use it and it works perfectly. Good way to set aside time to crank out work, then take a five minute break to recharge when the timer is done.

Is the speed "inconsistent" as in sometimes a specific page is fast and sometimes it's slow? Or are the same elements always slow? Perhaps you've already done this, but run the front-end through some tests on Pingdom first: http://tools.pingdom.com/fpt/ That's where I'd start if it was one of my client sites. From there I'd run the P3 plugin to see if there was a specific plugin causing any issues (https://wordpress.org/plugins/p3-profiler/). I have a few other things I check for but these first two are easy enough for a non-tech to at least get started.

In addition to what everyone else has already shared, make sure you sign up to receive daily HARO emails (http://www.helpareporter.com/) which contain requests from press nationwide who are looking for specific stories, experts, etc. You never know, someone might be putting together a major piece that is relevant to the product you are about to launch.