If you check the 'Keep ASM source files' check box in TailBite Manager, TailBite will make a copy of the intermediate ASM files used to make your library and put it in the library sources folder, which you can select below.
You can modify these ASM files and remake the library by processing the .Desc file instead of the original PureBasic source file. This allows you to optimize the code -if you know ASM, of course-.
Common optimizations would be removing unnecessary push/pop for unused registers, replace memory addresses for registers in loops, or replace some calls to PureBasic functions with your own specific code. For example, you may want to replace
PeekL() calls with faster and direct mov instructions.
Anyway, as you can use inline ASM in the original source code, optimizations will probably be limited to enter/leave procedure code, it's your choice.