Should Linq be avoided because its slow?
No. It should be avoided if it is not fast enough. Slow and not fast enough are not at all the same thing!
Slow is irrelevant to your customers, your management and your stakeholders. Not fast enough is extremely relevant. Never measure how fast something is; that tells you nothing that you can use to base a business decision on. Measure how close to being acceptable to the customer it is. If it is acceptable then stop spending money on making it faster; it’s already good enough.
Performance optimization is expensive. Writing code so that it can be read and maintained by others is expensive. Those goals are frequently in opposition to each other, so in order to spend your stakeholder’s money responsibly you’ve got to ensure that you’re only spending valuable time and effort doing performance optimizations on things that are not fast enough.
You’ve found an artificial, unrealistic benchmark situation where LINQ code is slower than some other way of writing the code. I assure you that your customers care not a bit about the speed of your unrealistic benchmark. They only care if the program you’re shipping to them is too slow for them. And I assure you, your management cares not a bit about that (if they’re competent); they care about how much money you’re spending needlessly to make stuff that is fast enough unnoticably faster, and making the code more expensive to read, understand, and maintain in the process.