Max data transfer size/rate via USART?

Can you post your source?

Serial defaults to 9600 baud. I'd first try setting up Serial at 57600 or 115200 baud, like Serial1.setup(57600, {rx:B7,tx:B6}); (where B7 and B6 are the pins you're using).

Serial1.pipe() might be another option, but I haven't played with that.

Next, sending chunks on an interval would probably work, but isn't the most efficient or robust, since you are depending on the receiving Pico always being ready on time for the next chunk. Which it might not always: sometimes it may have another task on its plate.

A somewhat more robust, brute force method, is to do your own chunking. Instead of waiting a fixed interval between chunks, you send a chunk, then wait for the receiving device to send you an acknowledgment. Then you send the next chunk, etc. Before all this, you send something like sending 152\n, where 152 is the number of chunks you plan on sending. That way receiver knows when it's done. (Serial1.pipe() probably does exactly this, internally.)