As the link shows, the syntax section of the Vim manual offers suggestions to increase speed on slow computers for all syntax categories, tex being one of them. That does not mean all those categories are a problem in Vim itself.

However, I do not mean to deny your actual experiences; I have seen all sorts of performance-related craziness that doesn't make sense, and I fully believe you when you say you had slow editing with near-empty .vimrc. I just find it baffling; for instance, here is a sample of my using vim in an older tablet, with latex syntax highlighting, UltiSnips, builtin terminal and w3m browser without any lag:

https://i.redd.it/p5h7ongm51541.jpg

So, baffling discrepancies; I wish software performance was more predictable.