Pxx is the main release. Under that there are subreleases (bugfixes). For P20 we have v.20.00.00.00 (bugged) and v.20.00.02.00 (flawless).
Under P20 I'm getting over 2 GB/s transfer speeds on my 4xSSD RAID0, so... I think 'slow' could be unappropriate term....
About your instructions, they are fine, BUT on some cards could be safer crossflashing with P11 at first and only THEN (simply) update to P19/P20...
Anyway, as you report, this is not ever needed...