Website loading very slowly even after troubleshooting

Hi all,

I've been recently helping a friend put together a website for their business and they are reporting the page is loading slowly for them. I created a new theme from underscores_me.

The website can be found here if anyone is able to take a look at it.

It is also slow for me too, especially the dashboard, but I am currently overseas with access to very poor internet so not much help diagnosing. In particular it seems horribly slow to upload new media, even when the images are only 50kb or so.

Google's Page Speed insights gives me a score of 85% but pingdom speed test has it at about 3.5 - 4.5 seconds which seems quite slow.

I have utilised all suggestions; W3 Cache, image optimisation, lazy load, removing unused plugins, deactivating all plugins to test improvements (none), I tried switching themes and there did seem to be marginal improvement.

I would be somewhat surprised if it were the hosting company, as it previously ran quite quickly with a different theme. This is what is making me think it may be my code but I have no idea what to look for as I've never experienced this issue before and am a beginner. I'm even worried it may be a security issue.

(edit) one more thing, I ran the speed test again (even worse this time, 6 sec, ugh), and I noticed it seems like I'm getting a lot of wait time (2.5s) trying to reach .com/undefined which obviously doesn't exist. See here: <img src="" title="source:" />

Any help would be so appreciated and I'm most happy to provide more details if need be. Thanks in advance for your assistance.