I’m going to disagree with you that idiomatic C# is somehow unnecessarily “slow”.

The performance overhead of a virtual vs non-virtual call on the CLR is negligible - same as with C++. For proof, look at Direct3D which is performance-critical, yet Direct3D’s API is based on COM which is all about vtable-calls.

Linq itself is also very decent. I won’t argue that Linq is perfect (e.g. suboptimal list allocations when the total output length can be known in-advance), but its hardly “slow” - certainly not enough to impact any production workloads - you’d only observe a difference between hand-written loops and a Linq expression in a contrived synthetic benchmark.