If you add extra bodies in any bit of communication, that communication tends to get delayed. I've seen this in corporate work environments on many occasions. Each body in the middle will have their own view of the matter and provide their own feedback accordingly. The little bits of constant back-and-forth in between different people add up the time it takes for a message to get through at the end of the day.
Example (with a bunch of hypotheticals based on my experiences in a corporate work environment; get ready for a block of text):
- United give their demands to our third-party representative via, say, email (15 million Euros).
- The email is read after, say, one hour (because they're busy with other matters as well).
- Our rep then sends an email back stating that Porto may not accept that and that they'd accept something like 25 million Euros. That email is read by United after some time (because United are also busy with work and not just twiddling their thumbs). Whoever's negotiating this on our end comes up with a counter-proposal that is then sent up to Judge, who then provides his feedback. After some more work, the proposal is sent to Woodward. After some more work, the proposal is sent to Joel Glazer. After some time (and updates via feedback), Joel approves on a new proposal.
- We then send this new proposal over to our rep. Rinse and repeat this until we come up with something that the rep's happy enough with.
- Our rep then sends this bit of info over to Porto's rep via email. Porto's rep read the email after some time as well.
- Porto's rep comes back and then provides their feedback. We go back into the same communication loop highlighted previously. Rinse and repeat until Porto's rep is happy with something.
- Porto's rep then sends this to the club. The club provides their feedback, and the info is eventually passed all the way back to United. Rinse and repeat until we come up with a deal that Porto are happy with.
- We finally submit a formal offer to Porto, which they'd accept once they see said offer.
If you want a footballing example of this, see the Koulibaly transfer saga. Napoli and City aren't directly dealing with each other; both clubs are going through Koulibaly's agent, and that's slowing down the talks.
Contrast this to a situation without intermediaries. United send an offer directly to Porto. Porto acknowledge the offer. Porto then goes through the deal on their end and provides feedback to United. United then offer a counter-proposal (after some internal assessments). Rinse and repeat until Porto are happy with something that we offer.
Why does going to agents take longer? Agents themselves have some idea of what the other party could accept, and we'd need to come to an agreement with them before something gets back to the club. If we negotiate directly with the club, however, we'd get instant feedback of what they'd want instead of what agents think they'd want. Fewer bodies => more direct and transparent communication => fewer delays
Hopefully that explains it. If not, then let me know what's contentious for you.