It's because one of their paid advertisers is pushing through mobile ads. They don't load on a PC, so the ad just keeps trying to push it through over and over again, killing your bandwidth and building up data in your cache. Right click on your back button. Do you remember those endless advertisement pages it shows?
The ads here push through so much spam it's not even useable on a phone if you're a free member. I tested it on multiple devices too, so it's not malware. Then I came here to ask about it, and what do you know. Shit tons of mobile adware coming through cache that don't display on a desktop due to formatting, but are still there. Either the forum is allowing it and making big money, or their "sponsored" advertisers are pulling a fast one on them.
This is why the forum is slow for you, and can't even process a post before timing out. That, and the unending advertisement data the site pushes through to your system cache compromising your actual computer performance too.
Anything for a buck I guess.