Best to Burn a CD-R at 1X or 2X Speed?

From what I understand the slower the better, but I read somewhere that writting at 1X speed can cause problems, is that true?

Slower is often better, but not always.
Depends on the media and burner.

CDR’s currently sold as compatible with 1x - 48x (or even 52x) are really more optimized for higher speed burns.
They may burn at 1X or 2X but, compared to much older (eg 4x) media, the burn quality might not be as good.

If you’re burning audio CD’s (I presume that is the basis for this question) then jitter and speed stability become issues too. Typically modern burners have the most rotational stability towards the upper end of their rated speed. So if you burn at 1X, there might be sufficient jitter introduced that some audio CD players will have trouble tracking.

I should mention I am using CD-Rs that are 1-8X and 1-6X 74 Minutes, stuff sold at the end of the 1990’s, no modern CD-R and no modern CD-R Writers either.

Within that set of media and equipment, you may very well get better results at 2X.

In that same time frame, I definitely owned cars, for instance, whose CD players would choke on discs burned at 4X and above.

The specific equipment would be:

Yamaha CRW-F1 CD Writer

TDK Made in Japan CD-R74 97m32s00f 1-6X (1997) and 1-8X (1998-1999).

Knowing that, better to write at 1X speed or 2X speed?

The objective way to find out is to do burns at different speeds using that type of media, and then scan with a drive that returns diagnostic information (LiteOn, Benq, etc) as detailed in many threads here.