And they promised that users of the free version would not be limited. Telegram found confirmation of reduced speed without a Premium subscription

This is mentioned in the source code

Telegram developers are going to reduce the download speed for users without a paid subscription, as reported by the Telegram channel «Dvach».

In the source code of the Telegram application, it was discovered that the messenger will reduce the speed of downloading and uploading files for users without a paid subscription if they exceed the preset traffic limit.

Details about this limit are not reported, but it is already known that the speed is planned to be limited to 10 Mbit/s for 60 minutes (and so on every time the limit is repeatedly exceeded).

At the time of the announcement of Telegram Premium, Pavel Durov promised that all functions of the messenger would not be limited for users of the free version.