Mark misunderstand webfarm to webform in his first reply.
Just like we ever research result, the session mode cannot be InProc, because we cannot interfere the synchronization of webfarm. So the only solution to make your pages works well is save the session in a server.
The webfarm is very old tech and had stopped to upgrade after 2012, we even cannot install it again now. So if the slow problem is caused by webfarm itself, I think we cannot solve it. For make sure that we need you do a simple test in your side: Please use your same page without webfarm environment, you can make it work in a normal IIS and just save the session in same server, and then access it to compare the load speed. If the speed is the same slow, that should be the page and session problem, we can go on look into it and see how to solve it. If the speed is fast in this scenario, that means the problem is caused by webfarm, I think maybe you want to find another solution to replace webfarm.