I am working on a hobby project using I2C for an OLED display and SPI for an SD card reader plus 3 digital potentiometers.

For the Teensy 4, the SD reader does not initialize. It's one of those super-cheap things with a 5-to-3.3v regulator and level shifters. I disabled the regulator on the card reader, and still no success. About to give up, I swapped my Teensy LC and witnessed the card reader able to initialize, read and write without a problem. But then I noticed the LC has problems with the OLED display. As per suggestion on the forum (thanks Frank), I boosted the pull-up resistors on the I2C pins and got the OLED display to work _better_ but still a little flaky. Of course I had to change timing on my loop() method on the LC but it's okay. (Did I forget to say I'm also receiving serial MIDI?)

So. Yes I am curious as what accounts for the differences here (seems like voltage/current more than timing or speed).

But mostly I'm trying to get a sense of which Teensy is right for my project. I would prefer to use a Teensy 4, but I'm not crunching numbers or driving the Audio Library. If the best advice is "don't be so cheap and get a better SD card reader", I'll take my medicine and do that.