The amount of slowdown exhibited should mostly depend on the indexes you have on the table. Each record inserted will cause each index for that table to be updated, thereby creating more and more slow down as your table grows.
There are several ways to maintain performance over the long haul, the simplest being have your fresh data written to a MySQL Memory table for insertions and queries, then have a cronjob transfer the data to a permanent table.
There's also message queueing (ZeroMQ, RabbitMQ, etc) or a NoSQL database (MongoDB)