It is certainly possible to write slow code without LINQ, all I'm saying is that I wouldn't blindly trust a blog post that talk about performance and use LINQ.
The article contents suggest deep understanding of the topic.
This type of thinking ("LINQ bad" or "SOLID good") is one reason among many why bad patterns proliferate through the projects e.g. "hey you should rewrite this code with SOLID principles in mind" (without accounting for the context) or "This code calculates the sum using LINQ, you should rewrite it" (LINQ's Sum implementation uses SIMD and is hard to beat).
This type of thinking ("LINQ bad" or "SOLID good") is one reason among many why bad patterns proliferate through the projects e.g. "hey you should rewrite this code with SOLID principles in mind" (without accounting for the context) or "This code calculates the sum using LINQ, you should rewrite it" (LINQ's Sum implementation uses SIMD and is hard to beat).