You really need to use common sense. On the low bands where you have deep long fading high speed CW can help. On the other hand sending high speed can get your report through before the fading gets your signal.
The same goes for heavy QRN, high speed can get the message through. Many peoples radios have poor group delay in their CW filters which stretches QRN out which entirely blanks your signal. Transmitting fast CW when heavy QRN is around can help. The bottom line is just to use a common sense speed of 25 to 30 wpm. If you detect the other operating is transmitting at high speed he can probably copy you at high speed, slowing down will break his rhythm causing confusion and broken calls because of the QRN or fading.
There is really no right or wrong answer you just learn from experience.
The most important rule is that you dont stretch out and slow send what you trying to send, this causes more errors because its hard to work out what you trying to send. THis is the worst possible thing you can do and it really ruins the receiving stations rhythm causing more errors. Send in a consistent manner and just repeat in a normal fist and speed. Thats how the PRO's use to do it on the commercial HF links
I did read somewhere, where a CW op was having trouble making the contact at his normal speed around 25+wpm, so dropped the speed down to 18 wpm and was about to finish off the QSO...simply because the signal was making the distance better.
Are the fast Op's shooting themselves in the foot so to speak by wanting to operate at a fast wpm speed and thus sacrifice a possible longer distance communication...
Assuming the OP is interested on making contacts around the world and just not local....some are, some are not..