How bad is the video performance on the Amstrad PC1512 and 1640?

[Trixter]

Does anyone know just how bad this actually is?

You can check yourself with TOPBENCH as there is a "vidram" performance metric that tests raw read/write speed to video RAM. Add your system to the database, then browse other systems (like "IBM 5150" with a CGA card) and check the vidram numbers.

The only caveat is that the test is done while in text mode. On real IBM CGA, that doesn't matter, but on clones and integrated compatibles, text mode might have different performance characteristics than graphics mode.