Strider movement speed was increased in 20w14a, but there now appears to be a problem with how they interact with normal blocks.

Whenever you find a Strider, it's spawned with its legs sunken slightly in the lava. This allows them to have their lava-walking speed. However, when you lead them over normal blocks such as Netherrack or magma blocks, their speed decreases dramatically. This part is intentional, of course, but what isn't intentional (to my knowledge) is that when you then lead them over lava once more after walking over the ground, their speed remains the same as if they were still on the ground, and their legs do not sink back into the lava.

I've tested this using the fungus-on-a-stick item as well, using it as a boost both before and after leading the Strider onto the ground. The boost the fungus-on-a-stick gives does not give the Strider it's normal lava speed back, even after being led onto the lava again from the normal ground.

I've included a picture showing the difference between a freshly-spawned Strider (the one without the saddle), and the one I've tested this behavior with (the one I'm sitting on). As you can see, the legs of the Strider my avatar has mounted is now standing above the lava, rather than in it.