SPI frequency setting is not accurate in Portenta H7

For example the following statement
SPI.beginTransaction (SPISettings (1000UL * 1000, MSBFIRST, SPI_MODE0)) ;
should set the SPI to 1MHz but the actual measured frequency is 631KHz

with the following settings
SPI.beginTransaction (SPISettings (4000UL * 1000, MSBFIRST, SPI_MODE0)) ;
SPI is set to 4MHz but the actual measured frequency is 2.4MHz

Start by reproducing the reported Portenta H7 behavior with the two SPI.beginTransaction(SPISettings(...)) examples and compare the measured frequencies with 1 MHz and 4 MHz. No source file or test is named; done means the configured SPI frequencies match the requested values, with a regression test if the project provides one.

Written by the indexing model from the issue text.