Foreword (Alternate Solutions - You must still have Optifine)
I have come up with two solutions for your problem in the case that the above does not work. I hope that one of these solutions will inevitably help you. I am almost positive that one or both of these solutions will be able to help you indefinitely if you follow the instructions step by step if you have any more technical problems make sure to address them openly as you did this time, this is a good way to receive public help. Please update me on whether one of these solutions worked for you. You could try both solutions or just one or the other, neither of these will harm your performance in any way and are entirely safe I will be watching this thread and good luck, Please, keep me updated!
Solution 1
If you have Optifine and its still not working there's a chance that it could be from not having enough ram allocated towards Minecraft, I'd only guess that possibly you're using an older computer, but if that's not the case then I am unsure of what to tell you because it should be working fine. Of course, it wouldn't hurt to try as allocating ram can speed up many things as well, Here's a video link on how to allocate more ram into your Minecraft. You may want to try this first and see when you join with the resource pack enabled if it will allow you to finally pass making the request as it will boost performance, Remember you will still need to be using optifine.
https://www.youtube.com/watch?v=Ng2t4sU53vg
Solution 2
The second solution could be to try and manually download the resource pack and use it that way instead of enabling it and joining the server, Optifine will still be needed for this. Downloading the servers resource pack manually is really simple and easy to do, Click this link for a video tutorial. The only problem with this is when you download the resource pack it will forever be stuck the way it is, but you can always just re-download with this solution to get an updated version.
https://www.youtube.com/watch?v=Vd_DP8lYDAk