I've noticed that 8250 UART based serial port performance is poorer in 2.5.x than 2.4.x and 2.2.x, on a couple of my machines.
The 486 SX-20 with 4 MB RAM, running 2.2.21 reliably achieves about 650 BPS download from another machine, with the port runnnig at 9600 bps. With 2.5.40, many characters are lost at 9600, making, e.g. a ZModem transfer retry for almost every block.
A 486 SX-25 with 8 MB RAM, running 2.4.19 manages about 950 BPS reliably with the port set at 9600 bps. With 2.5.40, there are again a lot of lost characters.
I know these are ancient machines, with rediculously low amounts of memory, but surely 9600 bps should be reliable, even if performance drops to 600-700 BPS, or even lower.
I originally thought that the new kernel was using up memory that was previously available to be used as a buffer, and that extra hard disk access was causing the lost characters, but this doesn't seem to be the case.