I'm weening off the default 27.6.1 in Xenial and leaning towards Seamonkey. I just unloaded 28.3.1 with the same performance hiccups.
...Pale Moon is based on much older code. It won’t have the new web features or performance improvements of modern versions of Firefox, nor does it support watching certain kinds of video with DRM.
Besides, Pale Moon tends to perform worse on browser benchmarks compared to modern browsers, which isn’t surprising given its age. The developer disagrees with browser benchmarking, but it’s not surprising a browser based on four year old code might be slower than a modern one.
The italic is exactly what I'm experiencing.