For a task like this, a language that supports multithreading and has good support for network programming would be a good choice. Some options to consider are:

1. Python: Python is a popular language for network programming and has good support for multithreading through its threading and concurrent.futures modules. It also has several libraries for SSH communication, including paramiko and ssh2-python.

2. Go: Go is a modern language designed for high-performance network programming and concurrency. Its native support for concurrency through goroutines and channels makes it easy to write concurrent code, and its standard library includes packages for SSH communication.

3. Rust: Rust is a systems programming language that emphasizes performance, safety, and concurrency. It has excellent support for multithreading and networking, and its ownership and borrowing system ensures that concurrent code is safe and free of data races.

All three of these languages would be suitable for implementing a fast, multithreaded program that attempts to login to multiple IP addresses over SSH. The choice of language ultimately depends on your personal preference and familiarity with the language, as well as any specific performance or security requirements of your project.
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