Keylight in AE renders very slow

If you use Dynamic Link to create a comp in After Effects and you keep the linked comp in the Premiere sequence, it is always going to take longer to render than it would to render your comp in After Effects and replace the footage in Premiere Pro. It will take longer because the Premiere Pro/Dynamic Link workflow will open a copy of After Effects in the background and render those frames utilizing only a small portion of your system resources. For this reason, the only time I keep Dynamic Link active in Premiere Pro is when the comp I'm creating will render at the rate of several frames per second. A keying composite will always take longer than that to render. A very complex dynamically linked composition can easily take up to 10 times as long to render in. Premiere Timeline as it would render the same comp using the Render Queue.

If you are just simply removing the background and your new background is added in Premiere Pro, you can achieve very good results in Premiere Pro using UltraKey. It's as good as any chroma key setup for any news broadcast.

If you are trying to do full simulated reality composites as they do in feature films, then you'll need to complete the entire comp in After Effects. If I had an hour and a half of made for tv quality green screen footage to prepare in After Effects I would probably want about 30 days to do the project. If I were doing keying as they do for TV news or even a TED talk, I would stick to Premiere Pro and be done with it in just as little more time than it took to cut the video.

If you must stick with After Effects, do not send the renders to the Media Encoder. Use the Render Queue to render a visually lossless DI (digital intermediate) using a good production format like ProRez or DNxHR and I'd make it 10 bit or better. It will render faster than anything you can render in the Media Encoder and you won't get color or compression artifacts building up like you would if you use the AME to render an MP4. 

Once again, using Dynamic Link is a good way to bring your clips into After Effects for further editing, but if the comps are complex and won't render at the rate of several frames per second, you are always better off rendering in After Effects to a suitable production format, then replacing the linked comp in Premiere with rendered footage. 

Yes dynamic link might be more efficient. I would have the clips in your Premiere timeline and then right click on the clip that you want to work on in AE, choose replace with After Effects Composition, when After Effects opens save the project. Do your keying and save the AE project. It should update in Premiere. If not press return to render in Premiere