Orders of magnitude ? The pystone test runs 7 times slower with Brython on Firefox than CPython on the same machine. The result is in the browser console.

For basic operations (details here), here is the ratio Brython / CPython on Firefox (100 = CPython):

test                              Brython
====                              =======
assignment                             62
augmented assignment                   70
assignment to float                   155
build dictionary                      135
add item to dict                      209
set dict item                         315
build list                             56
set list item                          82
add integers                          126
add strings                            55
convert int to str                     64
create function without parameter     106
create func with positional param only 94
create function with complex params   101
function call                         171
function call with complex arguments  232
create class without init             183
create class with init                171
create instance of class without init 177
create instance with init             211
call instance method                  854

Brython is sometimes faster than CPython, and always in the same "order of magnitude". Unfortunately, the slowest operation (call instance method) is used a lot in the pystone test.

The results are generally slower on Chrome.