Let’s start by stopping using Tumblr for a while. Done? Let’s do that a while longer. You are probably addicted to it. Maybe it’s not so much addicted as being overly obsessed by it. That might affect how you see the load times. The more you used it, the more you create what you perceive how fast anything in Tumblr should load. The Tumblr itself might not change, but sure does the illusion it gives you. Maybe you never knew how fast it actually is anyway. Time is relative, it is different to everyone. Slow to you, but fast enough to another. You know the feeling where you suddenly feel slower when stepping off a travelator and back to the non-moving floor? That is what time is. So you are psychologically programmed to how you personally perceived time, and now that it is only seemingly slower to you.
Joking aside, the problem might lies in the hardware and software. If the device you are using is old or running low on memory, naturally it would take a longer time to run Tumblr. It’s about time to replace or upgrade your device. Tumblr would start to load slower if your device is overwhelmed by all the contents. It does have a lot of content to process, ranging from gifs, videos, music and of course pictures. It could be the internet connection too, check that as well, make sure it’s properly set up to your devices. If the problem is in the software, then just be patient and wait for the fix.
You might try to send a message to Tumblr staff team, make them aware of the problem you are having. Don’t forget that if more people are facing the same problems, they are more likely to deal with it first. Well, basically this is all what you can do when you don’t know why is Tumblr so slow.