My advice is to use two physical hosts if you really want to use FT (especially if your degree is on the line). Nested virtualization is always going to be slower than bare metal, and that's especially true when you're doing something as intensive as FT. Get a second host and run ESXi on both directly.
I can't have a second host, the place where I work has only one host for me
How does the physical hardware look like, and how the virtual ESXi hosts regarding resources (CPU, RAM, ...), and the configuration (networking, storage, ...).
Remember that FT initially replicates the whole VM, which consumes resources. I the network, or the underlying storage becomes exhausted, thing may get really slow, which could explain what you experience.
Maybe you just need to remain patient until the initial synchronization is done!?